Semiconductor device and system using the same

ABSTRACT

To provide a semiconductor device which can execute the product-sum operation. The semiconductor device includes a first memory cell, a second memory cell, and an offset circuit. First analog data is stored in the first memory cell, and reference analog data is stored in the second memory cell. The first memory cell and the second memory cell supply a first current and a second current, respectively, when a reference potential is applied as a selection signal. The offset circuit has a function of supplying a third current corresponding to a differential current between the first current and the second current. In the semiconductor device, the first memory and the second memory supply a fourth current and a fifth current, respectively, when a potential corresponding to second analog data is applied as a selection signal. By subtracting the third current from a differential current between the fourth current and the fifth current, a current that depends on the sum of products of the first analog data and the second analog data is obtained.

TECHNICAL FIELD

One embodiment of the present invention relates to a semiconductordevice and a system using the semiconductor device.

Note that one embodiment of the present invention is not limited to theabove technical field. The technical field of the invention disclosed inthis specification and the like relates to an object, a method, or amanufacturing method. In addition, one embodiment of the presentinvention relates to a process, a machine, manufacture, or a compositionof matter. Specifically, examples of the technical field of oneembodiment of the present invention disclosed in this specificationinclude a semiconductor device, a display device, a liquid crystaldisplay device, a light-emitting device, a power storage device, animaging device, a memory device, a processor, an electronic device, amethod for driving any of them, a method for manufacturing any of them,a method for testing any of them, and a system including any of them.

BACKGROUND ART

An artificial neural network is an information processing system modeledon a biological neural network. A computer having a higher performancethan a conventional Neumann computer is expected to be provided byutilizing the artificial neural network, and in these years, a varietyof researches on an artificial neural network formed over an electroniccircuit have been carried out.

In the artificial neural network, units which resemble neurons areconnected to each other through units which resemble synapses. Bychanging the connection strength, a variety of input patterns arelearned, and pattern recognition, associative storage, or the like canbe performed at high speed. Furthermore, Non-Patent Document 1 disclosesa technique relating to a chip having a self-learning function with theartificial neural network.

REFERENCE

-   [Non-Patent Document 1] Yutaka Arima et al., “A Self-Learning Neural    Network Chip with 125 Neurons and 10K Self-Organization Synapses”,    IEEE Journal of Solid-State Circuits, Vol. 26, No. 4, April 1991,    pp. 607-611

DISCLOSURE OF INVENTION

In order to form an artificial neural network using a semiconductordevice, a synapse circuit that stores a connection strength between afirst neuron circuit and a second neuron circuit and performs aproduct-sum operation in which output of the first neuron circuit andthe connection strength are multiplied and summed needs to be provided.In other words, a memory that holds a connection strength, a multipliercircuit and an adder circuit that perform a product-sum operation, andthe like are necessarily mounted on the semiconductor device.

In the case where the memory, the multiplier circuit, the adder circuit,and the like are formed using digital circuits, the memory needs to beable to store multi-bit data and moreover, the multiplier circuit andthe adder circuit need to be able to perform multi-bit arithmeticoperation. In other words, a large-scale memory, a large-scalemultiplier circuit, and a large-scale adder circuit are required to forman artificial neural network using digital circuits; therefore, the chiparea of the digital circuits is increased.

An object of one embodiment of the present invention is to provide anovel semiconductor device. Another object of one embodiment of thepresent invention is to provide a module including the novelsemiconductor device. Another object of one embodiment of the presentinvention is to provide an electronic device using the module includingthe novel semiconductor device. Another object of one embodiment of thepresent invention is to provide a system with the module including thenovel semiconductor device.

Another object of one embodiment of the present invention is to providea semiconductor device with a small circuit area. Another object of oneembodiment of the present invention is to provide a semiconductor devicewith low power consumption. Another object of one embodiment of thepresent invention is to provide a novel semiconductor device having anauthentication function. Another object of one embodiment of the presentinvention is to provide a novel semiconductor device which compressesvideo data and/or image data.

Note that the objects of one embodiment of the present invention are notlimited to the above objects. The objects described above do not disturbthe existence of other objects. The other objects are the ones that arenot described above and will be described below. The other objects willbe apparent from and can be derived from the description of thespecification, the drawings, and the like by those skilled in the art.One embodiment of the present invention achieves at least one of theabove objects and the other objects. One embodiment of the presentinvention does not necessarily achieve all the above objects and theother objects.

(1)

One embodiment of the present invention is a semiconductor deviceincluding a memory cell array and an offset circuit, where the memorycell array includes a first memory cell and a second memory cell, wherethe offset circuit includes a first constant current circuit, a secondconstant current circuit, first to third transistors, a first capacitor,a first wiring, a second wiring, a first output terminal, a secondoutput terminal, and a current mirror circuit, where the first constantcurrent circuit is electrically connected to the first wiring andconfigured to supply a first current to the first wiring, where a firstterminal of the first transistor is electrically connected to a firstterminal of the second transistor, where a gate of the first transistoris electrically connected to a second terminal of the second transistor,where the first terminal of the second transistor is electricallyconnected to the first wiring, where a first terminal of the thirdtransistor is electrically connected to the second terminal of thesecond transistor, where a first terminal of the first capacitor iselectrically connected to the gate of the first transistor, where thefirst wiring is electrically connected to the first output terminal,where the second constant current circuit is electrically connected tothe second wiring and is configured to supply a second current to thesecond wiring, where the second wiring is electrically connected to thesecond output terminal, where the current mirror circuit is configuredto output a third current corresponding to a potential of the secondwiring from the first wiring and the second wiring, where the firstmemory cell is electrically connected to the first output terminal, andwhere the second memory cell is electrically connected to the secondoutput terminal.

(2)

Another embodiment of the present invention is the semiconductor deviceaccording to (1), where the first constant current circuit includes afourth transistor, where the second constant current circuit includes afifth transistor, where each of the fourth transistor and the fifthtransistor includes a back gate, where a first terminal of the fourthtransistor is electrically connected to the first wiring, where a gateof the fourth transistor is electrically connected to the first terminalof the fourth transistor, where a first terminal of the fifth transistoris electrically connected to the second wiring, and where a gate of thefifth transistor is electrically connected to the first terminal of thefifth transistor.

(3)

Another object of one embodiment of the present invention is thesemiconductor device according to (1) or (2), where the current mirrorcircuit includes a sixth transistor and a seventh transistor, where afirst terminal of the sixth transistor is electrically connected to thefirst wiring, where a gate of the sixth transistor is electricallyconnected to the second wiring, where a first terminal of the seventhtransistor is electrically connected to the second wiring, and where agate of the seventh transistor is electrically connected to the secondwiring.

(4)

Another object of one embodiment of the present invention is thesemiconductor device according to any one of (1) to (3), where the firstmemory cell includes an eighth transistor, a ninth transistor, and asecond capacitor, where the second memory cell includes a tenthtransistor, an eleventh transistor, and a third capacitor, where a firstterminal of the eighth transistor is electrically connected to a gate ofthe ninth transistor, where a first terminal of the second capacitor iselectrically connected to the first terminal of the eighth transistor,where a first terminal of the ninth transistor is electrically connectedto the first output terminal, where a first terminal of the tenthtransistor is electrically connected to a gate of the eleventhtransistor, where a first terminal of the third capacitor iselectrically connected to the first terminal of the tenth transistor,and where a first terminal of the eleventh transistor is electricallyconnected to the second output terminal.

(5)

Another embodiment of the present invention is the semiconductor deviceaccording to any one of (1) to (4), where the first to eleventhtransistors have the same polarity.

(6)

Another embodiment of the present invention is the semiconductor deviceaccording to any one of (1) to (5), where at least one of channelformation regions in the first to eleventh transistors includes an oxidecontaining at least one of indium, an element M (M is any of aluminum,gallium, yttrium, or tin), and zinc.

(7)

Another embodiment of the present invention is a semiconductor waferincluding a region for dicing and a plurality of semiconductor devicesdescribed in any one of (1) to (6).

(8)

Another embodiment of the present invention is an electronic deviceincluding the semiconductor device described in any one of (1) to (6)and a housing.

(9)

Another embodiment of the present invention is the electronic deviceaccording to (8), including the semiconductor device according to anyone of (1) to (6) and being configured to perform processing such aspattern recognition and associative storage.

(10)

Another embodiment of the present invention is a biometricauthentication system including the electronic device according to (9)and a device configured to obtain biological information.

(11)

Another embodiment of the present invention is a video distributionsystem including the electronic device according to (9) and beingconfigured to encode a video data and to send an encoded video data.

According to one embodiment of the present invention, a novelsemiconductor device can be provided. According to another embodiment ofthe present invention, a module including the novel semiconductor devicecan be provided. According to another embodiment of the presentinvention, an electronic device using the module including the novelsemiconductor device can be provided. According to one embodiment of thepresent invention, a system with the memory device including the novelsemiconductor device can be provided.

Another embodiment of the present invention can provide a semiconductordevice with a small circuit area. Another embodiment of the presentinvention can provide a semiconductor device with lower powerconsumption. Another embodiment of the present invention can provide anovel semiconductor device having an authentication function. Anotherembodiment of the present invention can provide a novel semiconductordevice that encodes video data and/or image data.

Note that the effects of one embodiment of the present invention are notlimited to the above effects. The effects described above do not disturbthe existence of other effects. The other effects are the ones that arenot described above and will be described below. The other effects willbe apparent from and can be derived from the description of thespecification, the drawings, and the like by those skilled in the art.One embodiment of the present invention has at least one of the aboveeffects and the other effects. Accordingly, one embodiment of thepresent invention does not have the aforementioned effects in somecases.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an example of a semiconductor device.

FIG. 2 is a circuit diagram showing an example of an offset circuit inthe semiconductor device of FIG. 1.

FIG. 3 is a circuit diagram showing an example of an offset circuit inthe semiconductor device of FIG. 1.

FIG. 4 is a circuit diagram showing an example of an offset circuit inthe semiconductor device of FIG. 1.

FIG. 5 is a circuit diagram showing an example of a memory cell array inthe semiconductor device of FIG. 1.

FIG. 6 is a circuit diagram showing an example of an offset circuit inthe semiconductor device of FIG. 1.

FIG. 7 is a circuit diagram showing an example of a memory cell array inthe semiconductor device of FIG. 1.

FIG. 8 is a timing chart showing an operation example of a semiconductordevice.

FIG. 9 is a timing chart showing an operation example of a semiconductordevice.

FIGS. 10A to 10E are a flow chart, a perspective view of an electroniccomponent, and perspective views of a semiconductor wafer.

FIGS. 11A to 11D are perspective views illustrating examples ofelectronic devices.

FIG. 12 is a schematic view illustrating data transmission in abroadcast system.

FIG. 13 illustrates a structure example of a video distribution systemin a medical field.

FIGS. 14A to 14C illustrate an example of a structure of a touch panel.

FIGS. 15A and 15B illustrate a structure example of a pixel in a displaypanel of a touch panel.

FIGS. 16A and 16B are cross-sectional views illustrating a structureexample of a touch panel.

FIGS. 17A and 17B are cross-sectional views illustrating a structureexample of a touch panel.

FIGS. 18A to 18C are schematic views illustrating an example of a shapeof a reflective film of a display panel.

FIG. 19 is a block diagram illustrating a structure example of an inputportion.

FIG. 20 is a circuit diagram showing a pixel of a display portion.

FIGS. 21A to 21C are a top view and cross-sectional views illustrating astructure example of a transistor.

FIG. 22A is a top view and FIGS. 22B and 22C are cross-sectional viewsillustrating a structural example of a transistor.

FIG. 23A is a top view and FIGS. 23B and 23C are cross-sectional viewsillustrating a structure example of a transistor.

FIG. 24A is a top view and FIGS. 24B and 24C are cross-sectional viewsillustrating a structure example of a transistor.

FIG. 25A is a top view and FIGS. 25B and 25C are cross-sectional viewsillustrating a structure example of a transistor.

FIG. 26A is a top view and FIGS. 26B and 26C are cross-sectional viewsillustrating a structure example of a transistor.

FIG. 27A is a top view and FIGS. 27B and 27C are cross-sectional viewsillustrating a structure example of a transistor.

FIGS. 28A to 28C each illustrate an atomic ratio range of an oxide.

FIG. 29 illustrates an InMZnO₄ crystal.

FIGS. 30A to 30C are each a band diagram of a stacked structure in anoxide.

FIGS. 31A to 31E show structural analysis of a CAAC-OS and a singlecrystal oxide semiconductor by XRD and selected-area electrondiffraction patterns of a CAAC-OS.

FIGS. 32A to 32E show a cross-sectional TEM image and plan-view TEMimages of a CAAC-OS and images obtained through analysis thereof.

FIGS. 33A to 33D show electron diffraction patterns and across-sectional TEM image of an nc-OS.

FIGS. 34A and 34B show cross-sectional TEM images of an a-like OS.

FIG. 35 shows a change in crystal part of an In—Ga—Zn oxide induced byelectron irradiation.

BEST MODE FOR CARRYING OUT THE INVENTION

An “electronic device”, an “electronic component”, a “module”, and a“semiconductor device” are described. In general, an “electronic device”may refer to as a personal computer, a mobile phone, a tablet terminal,an e-book reader, a wearable terminal, an audiovisual (AV) device, anelectronic appliance, a household appliance, an industrial appliance, adigital signage, a car, or an electric appliance including a system, forexample. An “electronic component” or a “module” may refer to aprocessor, a memory device, a sensor, a battery, a display device, alight-emitting device, an interface device, a radio frequency (RF) tag,a receiver, or a transmitter included in an electronic device. A“semiconductor device” may refer to a device including a semiconductorelement or a driver circuit, a control circuit, a logic circuit, asignal generation circuit, a signal conversion circuit, a potentiallevel converter circuit, a voltage source, a current source, a switchingcircuit, an amplifier circuit, a memory circuit, a memory cell, adisplay circuit, a display pixel, or the like which includes asemiconductor element and is included in an electronic component or amodule.

In this specification, an oxide semiconductor is referred to as an OS insome cases. Thus, a transistor including an oxide semiconductor in achannel formation region is referred to as an OS transistor in somecases.

Embodiment 1

In this embodiment, a product-sum operation circuit which is asemiconductor device according to one embodiment of the presentinvention will be described. Note that in the product-sum operationcircuit, the sum of products of first analog data and second analog datais conducted.

Configuration Example

FIG. 1 shows an example of a semiconductor device of one embodiment ofthe present invention. FIG. 1 is a block diagram of a product-sumoperation circuit. A semiconductor device 100 includes an offset circuit110 and a memory cell array 120.

The offset circuit 110 includes column output circuits OUT[1] to OUT[n](n is an integer greater than or equal to 1) and a reference columnoutput circuit C_(ref).

In the memory cell array 120, m (m is an integer greater than or equalto 1) memory cells AM are arranged in the column direction and n memorycells AM are arranged in the row direction; that is, m×n memory cells AMare provided. In addition, m memory cells AM_(ref) are arranged in thecolumn direction. The total number of the memory cells AM and the memorycells AM_(ref) provided in matrix in the memory cell array 120 ism×(n+1). In particular, in the memory cell array 120 in FIG. 1, thememory cell AM positioned in an i-th row and a j-th column is denoted bya memory cell AM[i,j] (i is an integer greater than or equal to 1 andless than or equal to m, and j is an integer greater than or equal to 1and less than or equal to n), and the memory cell AM_(ref) positioned inthe i-th row is denoted by a memory cell AM_(ref)[i].

The memory cell AM retains a potential corresponding to the first analogdata, and the memory cell AM_(ref) retains a predetermined potential.Note that the predetermined potential is a potential necessary for theproduct-sum operation, and in this specification, data corresponding tothis predetermined potential is referred to as reference analog data insome cases.

The memory cell array 120 includes output terminals SPT[1] to SPT[n].

The column output circuit OUT[j] includes an output terminal OT[j], andthe reference column output circuit C_(ref) includes an output terminalOT_(ref).

A wiring ORP is electrically connected to the column output circuitsOUT[1] to OUT[n], and a wiring OSP is electrically connected to thecolumn output circuits OUT[1] to OUT[n]. The wiring ORP and the wiringOSP are wirings for supplying a control signal to the offset circuit110.

An output terminal SPT[j] of the memory cell array 120 is electricallyconnected to a wiring B[j].

The output terminal OT[j] of the column output circuit OUT[j] iselectrically connected to the wiring B[j].

The output terminal OT_(ref) of the reference column output circuitC_(ref) is electrically connected to a wiring B_(ref).

The memory cell AM[i,j] is electrically connected to a wiring RW[i], awiring WW[i], a wiring WD[j], the wiring B[j], and a wiring VR.

The memory cell AM_(ref)[i] is electrically connected to the wiringRW[i], the wiring WW[i], a wiring WD_(ref), the wiring B_(ref), and thewiring VR.

The wiring WW[i] functions as a wiring for supplying a selection signalto the memory cells AM[i,1] to AM[i,n] and the memory cell AM_(ref)[i].The wiring RW[i] functions as a wiring for supplying either a referencepotential or a potential corresponding to the second analog data to thememory cells AM[i,1] to AM[i,n] and the memory cell AM_(ref)[i]. Thewiring WD[j] functions as a wiring for supplying writing data to thememory cells AM in the j-th column. The wiring VR functions as a wiringfor supplying a predetermined potential to the memory cells AM or thememory cells AM_(ref) when data is read out from the memory cells AM orthe memory cells AM_(ref).

The wiring B[j] functions as a wiring for supplying a signal from thecolumn output circuit OUT[j] to the memory cells AM in the j-th columnin the memory cell array 120.

The wiring B_(ref) functions as a wiring for supplying a signal from thereference column output circuit C_(ref) to the memory cells AM_(ref)[1]to AM_(ref)[m].

In the semiconductor device 100 in FIG. 1, only the following componentsare shown: the offset circuit 110; the memory cell array 120; the columnoutput circuit OUT[1]; the column output circuit OUT[j]; the columnoutput circuit OUT[n]; the reference column output circuit C_(ref); anoutput terminal OT[1]; the output terminal OT[j]; an output terminalOT[n]; the output terminal OT_(ref); an output terminal SPT[1]; theoutput terminal SPT[j]; an output terminal SPT[n]; a memory cellAM[1,1]; the memory cell AM[i,1]; a memory cell AM[m,1]; a memory cellAM[1,j]; the memory cell AM[i,j]; a memory cell AM[m,j]; a memory cellAM[1,n]; the memory cell AM[i,n]; a memory cell AM[m,n]; a memory cellAM_(ref)[1]; the memory cell AM_(ref)[i]; a memory cell AM_(ref)[m]; thewiring OSP; the wiring ORP; a wiring B[1]; the wiring B[j]; a wiringB[n]; the wiring Bra; a wiring WD[1]; the wiring WD[j]; a wiring WD[n];the wiring WD_(ref); the wiring VR; a wiring RW[1]; the wiring RW[i]; awiring RW[m]; a wiring WW[1]; the wiring WW[i]; and a wiring WW[m].Other circuits, wirings, elements, and reference numerals thereof arenot shown.

Note that a structure of one embodiment of the present invention is notlimited to the configuration of the semiconductor device 100 shown inFIG. 1. Depending on circumstances or conditions or as needed, thestructure of the semiconductor device 100 can be changed. For example,depending on a circuit configuration of the semiconductor device 100,the wiring WD[j] and the wiring VR are not separately provided, and onewiring may be shared therebetween. Alternatively, depending on a circuitconfiguration of the semiconductor device 100, the wiring ORP and thewiring OSP are not separately provided, and one wiring may be sharedtherebetween.

«Offset Circuit 110»

Next, an example of a circuit configuration that can be applied for theoffset circuit 110 is described. FIG. 2 shows an offset circuit 111 asan example of the offset circuit 110.

The offset circuit 111 is electrically connected to a wiring VDDL and awiring VSSL for supplying a power supply voltage. Specifically, each ofthe column output circuits OUT[1] to OUT[n] are electrically connectedto the wiring VDDL and the wiring VSSL, and the reference column outputcircuit C_(ref) is electrically connected to the wiring VDDL. Note thata current mirror circuit CM described later is electrically connected tothe wiring VSSL in some cases. The wiring VDDL supplies a high-levelpotential. The wiring VSSL supplies a low-level potential.

A circuit configuration of an inside of the column output circuit OUT[j]is described below. The column output circuit OUT[j] includes a constantcurrent circuit CI, transistors Tr1 to Tr3, a capacitor C1, and a wiringOL[j]. The current mirror circuit CM is shared between the column outputcircuits OUT[1] to OUT[n] and the reference column output circuitC_(ref).

The constant current circuit CI includes a terminal CT1 and a terminalCT2. The terminal CT1 functions as an input terminal of the constantcurrent circuit CI, and the terminal CT2 functions as an output terminalof the constant current circuit CI. The current mirror circuit CM sharedbetween the column output circuits OUT[1] to OUT[n] and the referencecolumn output circuit C_(ref) includes terminals CT5[1] to CT5[n],terminals CT6[1] to CT6[n], a terminal CT7, and a terminal CT8.

The constant current circuit CI has a function of keeping the amount ofcurrent flowing from the terminal CT1 to the terminal CT2 constant.

In the column output circuit OUT[j], a first terminal of the transistorTr1 is electrically connected to the wiring OL[j], a second terminal ofthe transistor Tr1 is electrically connected to the wiring VSSL, and agate of the transistor Tr1 is electrically connected to a first terminalof the capacitor C1. A first terminal of the transistor Tr2 iselectrically connected to the wiring OL[j], a second terminal of thetransistor Tr2 is electrically connected to the first terminal of thecapacitor C1, and a gate of the transistor Tr2 is electrically connectedto the wiring OSP. A first terminal of the transistor Tr3 iselectrically connected to the first terminal of the capacitor C1, asecond terminal of the transistor Tr3 is electrically connected to thewiring VSSL, and a gate of the transistor Tr3 is electrically connectedto the wiring ORP. A second terminal of the capacitor C1 is electricallyconnected to the wiring VSSL.

Note that each of the transistors Tr1 to Tr3 is preferably an OStransistor. In addition, each channel formation region in thetransistors Tr1 to Tr3 is preferably formed using an oxide containing atleast one of indium, an element M (M is any of aluminum, gallium,yttrium, or tin), and zinc. It is further preferable that thetransistors Tr1 to Tr3 have a structure of a transistor described inEmbodiment 5.

The OS transistor has a characteristic of extremely low off-statecurrent. Thus, when the OS transistor is in an off state, the amount ofleakage current flowing between a source and a drain can be extremelysmall. With use of the OS transistors as the transistors Tr1 to Tr3, theleakage current of each of the transistors Tr1 to Tr3 can be suppressed,which enables a product-sum operation circuit with high calculationaccuracy to be achieved in some cases.

In the column output circuit OUT[j], the terminal CT1 of the constantcurrent circuit CI is electrically connected to the wiring VDDL, and theterminal CT2 of the constant current circuit CI is electricallyconnected to the terminal CT5[j] of the current mirror circuit CM. Theterminal CT6[j] of the current mirror circuit CM is electricallyconnected to the output terminal OT[j].

Note that the wiring OL[j] is a wiring for making the terminal CT2 ofthe constant current circuit CI being electrically connected to theoutput terminal OT[j] through the terminal CT5[j] and the terminalCT6[1] of the current mirror circuit CM.

Next, the reference column output circuit C_(ref) is described. Thereference column output circuit C_(ref) includes the constant currentcircuit CI_(ref) and a wiring OL_(ref). As described above, thereference column output circuit C_(ref) includes the current mirrorcircuit CM that is shared with the column output circuits OUT[1] toOUT[n].

The constant current circuit CI_(ref) includes a terminal CT3 and aterminal CT4. The terminal CT3 functions as an input terminal of theconstant current circuit CI_(ref), and the terminal CT4 functions as anoutput terminal of the constant current circuit CI_(ref).

The constant current circuit CI_(ref) has a function of keeping theamount of current flowing from the terminal CT3 to the terminal CT4constant.

In the reference column output circuit C_(ref), the terminal CT3 of theconstant current circuit CI_(ref) is electrically connected to thewiring VDDL, and the terminal CT4 of the constant current circuitCI_(ref) is electrically connected to the terminal CT7 of the currentmirror circuit CM. The terminal CT8 of the current mirror circuit CM iselectrically connected to the output terminal OT_(ref).

The wiring OL_(ref) is a wiring for making the terminal CT4 of theconstant current circuit CI_(ref) being electrically connected to theoutput terminal OT_(ref) through the terminal CT7 and the terminal CT8of the current mirror circuit CM.

In the current mirror circuit CM, the terminal CT5[j] is electricallyconnected to the terminal CT6[j], and the terminal CT7 is electricallyconnected to the terminal CT8. In addition, a wiring IL[j] iselectrically connected between the terminal CT5[j] and the terminalCT6[j], and a wiring IL_(ref) is electrically connected between theterminal CT7 and the terminal CT8. Furthermore, a connection portion ofthe wiring IL_(ref) between the terminal CT7 and the terminal CT8 is anode NCM_(ref). The current mirror circuit CM has a function ofequalizing the amount of current flowing in the wiring IL_(ref) and theamount of current flowing in each of wirings IL[1] to IL[n] withreference to the potential at the node NCM_(ref).

In the offset circuit 111 in FIG. 2, only the following components areshown: the column output circuit OUT[1]; the column output circuitOUT[j]; the column output circuit OUT[n]; the reference column outputcircuit C_(ref); the constant current circuit CI; the constant currentcircuit CI_(ref); the current mirror circuit CM; the output terminalOT[1]; the output terminal OT[j]; the output terminal OT[n]; the outputterminal OT_(ref); the terminal CT1; the terminal CT2; the terminal CT3;the terminal CT4; the terminal CT5[1]; the terminal CT5[j]; the terminalCT5[n]; the terminal CT6[1]; the terminal CT6[j]; the terminal CT6[n];the terminal CT7; the terminal CT8; the transistor Tr1; the transistorTr2; the transistor Tr3; the capacitor C1; a wiring OL[1]; the wiringOL[j]; a wiring OL[n]; the wiring OL_(ref); the wiring ORP; the wiringOSP; the wiring B[1]; the wiring B[j]; the wiring B[n]; the wiringB_(ref); the wiring IL[1]; the wiring IL[j]; the wiring IL[n]; thewiring IL_(ref); the node NCM_(ref); the wiring VDDL; and the wiringVSSL. Other circuits, wirings, elements, and reference numerals thereofare not shown.

Note that a structure of one embodiment of the present invention is notlimited to the configuration of the offset circuit 111 in FIG. 2.Depending on circumstances or conditions or as needed, the structure ofthe offset circuit 111 can be changed.

[Constant Current Circuits CI and CI_(ref)]

Next, an example of internal structures of the constant current circuitCI and the constant current circuit CI_(ref) is described.

An offset circuit 112 shown in FIG. 3 is a circuit diagram showing anexample of internal structures of the constant current circuit CI andthe constant current circuit CI_(ref) included in the offset circuit 111shown in FIG. 2.

In the column output circuit OUT[j], the constant current circuit CIincludes a transistor Tr4. The transistor Tr4 has a dual gate structureincluding a first gate and a second gate.

Note that in this specification, the first gate in the transistor havinga dual gate structure indicates a front gate, and a term “first gate”can be replaced with a simple term “gate”. Besides, the second gate inthe transistor having a dual gate structure indicates a back gate, and aterm “second gate” can be replaced with a term “back gate”.

A first terminal of the transistor Tr4 is electrically connected to theterminal CT1 of the constant current circuit CI. A second terminal ofthe transistor Tr4 is electrically connected to the terminal CT2 of theconstant current circuit CI. A gate of the transistor Tr4 iselectrically connected to the terminal CT2 of the constant currentcircuit CI. A back gate of the transistor Tr4 is electrically connectedto a wiring BG[j].

In the reference column output circuit C_(ref), the constant currentcircuit CI_(ref) includes a transistor Tr6. The transistor Tr6 has adual gate structure including a gate and a back gate.

A first terminal of the transistor Tr6 is electrically connected to theterminal CT3 of the constant current circuit CI_(ref). A second terminalof the transistor Tr6 is electrically connected to the terminal CT4 ofthe constant current circuit CI_(ref). The gate of the transistor Tr6 iselectrically connected to the terminal CT4 of the constant currentcircuit CI_(ref). The back gate of the transistor Tr6 is electricallyconnected to a wiring BG_(ref).

In the above connection structure, the threshold voltages of thetransistor Tr4 and the transistor Tr6 can be controlled when a potentialis applied to the wiring BG[j] and the wiring BG_(ref).

Each of the transistor Tr4 and the transistor Tr6 is preferably an OStransistor. In addition, each channel formation region in thetransistors Tr4 and Tr6 is preferably formed using an oxide containingat least one of indium, the element M, and zinc. It is furtherpreferable that the transistors Tr4 and Tr6 have a structure of atransistor described in Embodiment 5.

With use of the OS transistors as the transistors Tr4 and Tr6, theleakage current of each of the transistors Tr4 and Tr6 can besuppressed, which enables a product-sum operation circuit with highcalculation accuracy to be achieved in some cases.

In the offset circuit 112 shown in FIG. 3, only the following componentsare shown: the column output circuit OUT[1]; the column output circuitOUT[j]; the column output circuit OUT[n]; the reference column outputcircuit C_(ref); the constant current circuit CI; the constant currentcircuit CI_(ref); the current mirror circuit CM; the output terminalOT[1]; the output terminal OT[j]; the output terminal OT[n]; the outputterminal OT_(ref); the terminal CT1; the terminal CT2; the terminal CT3;the terminal CT4; the terminal CT5[1]; the terminal CT5[j]; the terminalCT5[n]; the terminal CT6[1]; the terminal CT6[j]; the terminal CT6[n];the terminal CT7; the terminal CT8; the transistor TO; the transistorTr2; the transistor Tr3; the transistor Tr4; the transistor Tr6; thecapacitor C1; the wiring OL[1]; the wiring OL[j]; the wiring OL[n]; thewiring OL_(ref); the wiring ORP; the wiring OSP; the wiring B[1]; thewiring B[j]; the wiring B[n]; the wiring B_(ref); a wiring BG[1]; thewiring BG[j]; a wiring BG[n]; the wiring BG_(ref); the wiring IL[1]; thewiring IL[j]; the wiring IL[n]; the wiring IL_(ref); the node NCM_(ref);the wiring VDDL; and the wiring VSSL. Other circuits, wirings, elements,and reference numerals thereof are not shown.

[Current Mirror Circuit CM]

Next, an internal structure example of the current mirror circuit CMwill be described.

An offset circuit 113 shown in FIG. 4 is a circuit diagram of aninternal structure example of the current mirror circuit CM included inthe offset circuit 111 shown in FIG. 2.

In the current mirror circuit CM, each of the column output circuitsOUT[1] to OUT[n] includes a transistor Tr5, and the reference columnoutput circuit C_(ref) includes a transistor Tr7.

A first terminal of the transistor Tr5 in the column output circuitOUT[j] is electrically connected to the terminal CT5[j] and the terminalCT6[j] of the current mirror circuit CM. A second terminal of thetransistor Tr5 in the column output circuit OUT[j] is electricallyconnected to the wiring VSSL. A gate of the transistor Tr5 in the columnoutput circuit OUT[j] is electrically connected to the terminal CT7 andthe terminal CT8 in the current mirror circuit CM.

A first terminal of the transistor Tr7 in the reference column outputcircuit C_(ref) is electrically connected to the terminal CT7 and theterminal CT8 of the current mirror circuit CM. A second terminal of thetransistor Tr7 in the reference column output circuit C_(ref) iselectrically connected to the wiring VSSL. A gate of the transistor Tr7in the reference column output circuit C_(ref) is electrically connectedto the terminal CT7 and the terminal CT8 of the current mirror circuitCM.

In the above connection structure, a potential of the node NCM_(ref) canbe applied to the gate of the transistor Tr5 in each of the columnoutput circuits OUT[1] to OUT[n], and the amount of current flowingbetween a source and a drain of the transistor Tr7 can be equalized tothe amount of current flowing between a source and a drain of thetransistor Tr5 in each of the column output circuits OUT[1] to OUT[n].

Each of the transistor Tr5 and the transistor Tr7 is preferably an OStransistor. In addition, each channel formation region in thetransistors Tr5 and Tr7 is preferably formed using an oxide containingat least one of indium, the element M, and zinc. It is furtherpreferable that the transistors Tr5 and Tr7 have a structure of atransistor described in Embodiment 5.

With use of the OS transistors as the transistors Tr5 and Tr7, theleakage current of each of the transistors Tr5 and Tr7 can besuppressed, which enables a product-sum operation circuit with highcalculation accuracy to be achieved in some cases.

In the offset circuit 113 shown in FIG. 4, only the following componentsare shown: the column output circuit OUT[1]; the column output circuitOUT[j]; the column output circuit OUT[n]; the reference column outputcircuit C_(ref); the constant current circuit CI; the constant currentcircuit CI_(ref); the current mirror circuit CM; the output terminalOT[1]; the output terminal OT[j]; the output terminal OT[n]; the outputterminal OT_(ref); the terminal CT1; the terminal CT2; the terminal CT3;the terminal CT4; the terminal CT5[1]; the terminal CT5[j]; the terminalCT5[n]; the terminal CT6[1]; the terminal CT6[j]; the terminal CT6[n];the terminal CT7; the terminal CTB; the transistor TO; the transistorTr2; the transistor Tr3; the transistor Tr5; the transistor Tr7; thecapacitor C1; the wiring OL[1]; the wiring OL[j]; the wiring OL[n]; thewiring OL_(ref); the wiring ORP; the wiring OSP; the wiring B[1]; thewiring B[j]; the wiring B[n]; the wiring Bra; the wiring IL[1]; thewiring IL[j]; the wiring IL[n]; the wiring IL_(ref); the node NCM_(ref);the wiring VDDL; and the wiring VSSL. Other circuits, wirings, elements,and reference numerals thereof are not shown.

«Memory Cell Array 120»

Next, a circuit configuration example that can be employed in the memorycell array 120 is described. FIG. 5 shows a memory cell array 121 as anexample of the memory cell array 120.

The memory cell array 121 includes the memory cells AM and the memorycells AM_(ref). Each of the memory cells AM included in the memory cellarray 121 includes a transistor Tr11, a transistor Tr12, and a capacitorC2. The memory cells AM_(ref)[1] to AM_(ref)[m] each include thetransistor Tr11, the transistor Tr12, and the capacitor C2.

For the connection structure in the memory cell array 121, theexplanation is made with a focus on the memory cell AM[i,j]. A firstterminal of the transistor Tr11 is electrically connected to a gate ofthe transistor Tr12 and a first terminal of the capacitor C2. A secondterminal of the transistor Tr11 is electrically connected to the wiringWD[j]. A gate of the transistor Tr11 is electrically connected to thewiring WW[i]. A first terminal of the transistor Tr12 is electricallyconnected to the wiring B[j], and a second terminal of the transistorTr12 is electrically connected to the wiring VR. A second terminal ofthe capacitor C2 is electrically connected to the wiring RW[i].

In the memory cell AM[i,j], a connection portion of the first terminalof the transistor Tr11, the gate of the transistor Tr12, and the firstterminal of the capacitor C2 is a node N[i,j]. In one embodiment of thepresent invention, a potential corresponding to the first analog data isretained at the node N[i,j].

Next, the explanation is made with a focus on the memory cellAM_(ref)[i]. The first terminal of the transistor Tr11 is electricallyconnected to the gate of the transistor Tr12 and the first terminal ofthe capacitor C2. A second terminal of the transistor Tr11 iselectrically connected to the wiring WD_(ref). A gate of the transistorTr11 is electrically connected to the wiring WW[i]. A first terminal ofthe transistor Tr12 is electrically connected to the wiring B_(ref). Asecond terminal of the transistor Tr12 is electrically connected to thewiring VR. A second terminal of the capacitor C2 is electricallyconnected to the wiring RW[i].

In the memory cell AM_(ref)[i], a connection portion of the firstterminal of the transistor Tr11, the gate of the transistor Tr12, andthe first terminal of the capacitor C2 is a node N_(ref)[i].

Note that each of the transistor Tr11 and the transistor Tr12 ispreferably an OS transistor. In addition, each channel formation regionin the transistors Tr11 and Tr12 is preferably formed using an oxidecontaining at least one of indium, the element M, and zinc. It isfurther preferable that the transistors Tr11 and Tr12 have a structureof a transistor described in Embodiment 5.

With use of the OS transistors as the transistors Tr11 and Tr12, theleakage current of each of the transistors Tr11 and Tr12 can besuppressed, which enables a product-sum operation circuit with highcalculation accuracy to be achieved in some cases. Furthermore, with useof the OS transistor as the transistor Tr11, the amount of leakagecurrent from a retention node to a writing word line can be extremelysmall when the transistor Tr11 is in an off state. In other words,frequencies of refresh operation at the retention node can be reduced;thus, power consumption of a semiconductor device can be reduced.

Furthermore, when all of the above-described transistors Tr1 to Tr7,Tr11, and Tr12 are OS transistors, a manufacturing process of thesemiconductor device can be shortened. Thus, a time needed formanufacturing semiconductor devices can be shortened, and the number ofdevices manufactured in a certain time period can be increased.

Note that the transistor Tr1, the transistors Tr4 to Tr7, and thetransistor Tr12 operate in a saturation region unless otherwisespecified. In other words, the gate voltage, source voltage, and drainvoltage of each of the transistor Tr1, the transistors Tr4 to Tr7, andthe transistor Tr12 are appropriately biased so that the transistorsoperate in the saturation region. Note that even when the operations ofthe transistor Tr1, the transistors Tr4 to Tr7, and the transistor Tr12are deviated from ideal operation in a saturation region, the gatevoltage, source voltage, and drain voltage of each of the transistorsare regarded as being appropriately biased as long as the accuracy ofoutput data is within a desired range.

In the memory cell array 121 shown in FIG. 5, only the followingcomponents are shown: the memory cell AM[1,1]; the memory cell AM[i,1];the memory cell AM[m, 1]; the memory cell AM[1,j]; the memory cellAM[i,j]; the memory cell AM[m,j]; the memory cell AM[1,n]; the memorycell AM[i,n]; the memory cell AM[m,n]; the memory cell AM_(ref)[1]; thememory cell AM_(ref)[i]; the memory cell AM_(ref)[m]; the wiring RW[1];the wiring RW[i]; the wiring RW[m]; the wiring WW[1]; the wiring WW[i];the wiring WW[m]; the wiring WD[1]; the wiring WD[j]; the wiring WD[n];the wiring WD_(ref); the wiring B[1]; the wiring B[j]; the wiring B[n];the wiring B_(ref); the wiring VR; the output terminal SPT[1]; theoutput terminal SPT[j]; the output terminal SPT[n]; a node N[1,1]; anode N[i,1]; a node N[m,1]; a node N[1,j]; the node N[i,j]; a nodeN[m,j]; a node N[1,n]; a node N[i,n]; a node N[m,n]; a node N_(ref)[1];the node N_(ref)[i]; a node N_(ref)[m]; the transistor Tr11; thetransistor Tr12; and the capacitor C2. Other circuits, wirings,elements, and reference numerals thereof are not shown.

Depending on circumstances or conditions or as needed, the semiconductordevice of one embodiment of the present invention may have a combinedstructure of the above structure examples.

Operation Example

An example of operation of the semiconductor device 100 of oneembodiment of the present invention is described. Note that thesemiconductor device 100 described in this operation example includes anoffset circuit 150 shown in FIG. 6 as the offset circuit 110 and amemory cell array 160 shown in FIG. 7 as the memory cell array 120 ofthe semiconductor device 100.

The offset circuit 150 shown in FIG. 6 has a circuit configuration wherethe constant current circuit CI and the constant current circuitCI_(ref) of the offset circuit 112 in FIG. 3 and the current mirrorcircuit CM of the offset circuit 113 in FIG. 4 are used. With use of thestructure as shown in FIG. 6, all of the transistors in the offsetcircuit 150 can have the same polarity. For the description of thisoperation example, FIG. 6 shows the column output circuit OUT[j], acolumn output circuit OUT[j+1], and the reference column output circuitC_(ref).

In FIG. 6, Ic[j] denotes a current flowing from the first to secondterminal of the transistor Tr4 in the constant current circuit CI of thecolumn output circuit OUT[j], Ic[j+1] denotes a current flowing from thefirst to second terminal of the transistor Tr4 in the constant currentcircuit CI of the column output circuit OUT[j+1], and IC_(ref) denotes acurrent flowing from the first to second terminal of the transistor Tr6in the constant current circuit CI_(ref) of the reference column outputcircuit C_(ref). In the current mirror circuit CM, I_(CM) collectivelydenotes a current flowing to the first terminal of the transistor Tr5through the wiring IL[j] in the column output circuit OUT[j], a currentflowing to the first terminal of the transistor Tr5 through a wiringIL[j+1] in the column output circuit OUT[j+1], and a current flowing inthe transistor Tr7 through the wiring IL_(ref) in the reference columnoutput circuit C_(ref). Furthermore, I_(CP)[j] denotes a current flowingfrom the wiring OL[j] to the first terminal of the transistor Tr1 or Tr2in the column output circuit OUT[j], and I_(CP)[j+1] denotes a currentflowing from a wiring OL[i+1] to the first terminal of the transistorTr1 or Tr2 in the column output circuit OUT[j+1]. Moreover, I_(B)[j]denotes a current outputted from the output terminal OT[i] of the columnoutput circuit OUT[j] to the wiring B[j], I_(B)[j+1] denotes a currentoutputted from an output terminal OT[j+1] of the column output circuitOUT[j+1] to a wiring B[j+1], and I_(Bref) denotes a current outputtedfrom the output terminal OT_(ref) of the reference column output circuitC_(ref) to the wiring B_(ref).

The memory cell array 160 shown in FIG. 7 has a structure similar tothat of the memory cell array 121 shown in FIG. 5. For the descriptionof this operation example, FIG. 7 shows the memory cell AM[i,j], amemory cell AM[i+1,j], a memory cell AM[i,j+1], a memory cellAM[i+1,j+1], the memory cell AM_(ref)[i], and a memory cellAM_(ref)[i+1].

In FIG. 7, I_(B)[j] denotes a current inputted from the wiring B[j],I_(B)[j+1] denotes a current inputted from the wiring B[j+1], andI_(Bref) denotes a current inputted from the wiring B_(ref). Inaddition, ΔI_(B)[j] denotes a current outputted from the output terminalSPT[i] that is electrically connected to the wiring B[j], andΔI_(B)[j+1] denotes a current outputted from an output terminal SPT[j+1]that is electrically connected to the wiring B[j+1].

FIG. 8 and FIG. 9 are timing charts showing the operation example of thesemiconductor device 100. The timing chart in FIG. 8 shows changes inpotentials from Time T01 to Time T08 of the wiring WW[i], a wiringWW[i+1], the wiring WD[j], a wiring WD[j+1], the wiring WD_(ref), thenode N[i,j], a node N[i,j+1], a node N[i+1,j], a node N[i+1,j+1], thenode N_(ref)[i], a node N_(ref)[i+1], the wiring RW[i], a wiringRW[i+1], the wiring OSP, and the wiring ORP. This timing chart alsoshows the amount of changes in a current ΣI[i,j], a current ΣI[i,j+1],and a current I_(Bref) from Time T01 to Time T08. Note that the currentΣI[i,j] is a value of current flowing in the transistor Tr12 of thememory cell AM[i,j], which is obtained by summing over i from 1 to m,and the current ΣI[i,j+1] is the sum of the amounts of a value ofcurrent flowing in the transistor Tr12 of the memory cell AM[i,j+1],which is obtained by summing over i from 1 to m. The operation examplefrom Time T09 to Time T14 is shown in FIG. 9 as the rest of theoperation shown in the timing chart in FIG. 8. At and after fter TimeT09, the potentials of the wiring WW[i], the wiring WW[i+1], the wiringORP, and the wiring OSP are kept at a low level without any change, andpotentials of the wiring WD[l], the wiring WD[j+1], and the wiringWD_(ref) are kept at a ground potential without any change. Thus, in thetiming chart in FIG. 9, the changes in potentials of the wiring WW[i],the wiring WW[i+1], the wiring WD[i], the wiring WD[j+1], the wiringWD_(ref), the wiring ORP, and the wiring OSP are not shown. Furthermore,the timing chart in FIG. 9 shows variations in the amount of currentΔI_(B)[j] and the amount of current ΔI_(B)[i+1] described later.

«Period from Time T01 to Time T02»

During a period from Time T01 to Time T02, a high-level potential(denoted by High in FIG. 8) is applied to the wiring WW[i], and alow-level potential (denoted by Low in FIG. 8) is applied to the wiringWW[i+1]. Furthermore, a potential higher than the ground potential(denoted by GND in FIG. 8) by V_(PR)−V_(X)[i,j] is applied to the wiringWD[j], the potential higher than the ground potential byV_(PR)−V_(X)[i,j+1] is applied to the wiring WD[j+1], and a potentialhigher than the ground potential by V_(PR) is applied to the wiringWD_(ref). Moreover, a reference potential (denoted by REFP in FIG. 8) isapplied to the wiring RW[i] and the wiring RW[i+1].

The potential V_(X)[i,j] and the potential V_(X)[i,j+1] each correspondto the first analog data. The potential V_(PR) corresponds to thereference analog data.

In this period, a high-level potential is applied to the gates of thetransistors Tr11 in the memory cell AM[i,j], the memory cell AM[i,j+1],and the memory cell AM_(ref)[i]; accordingly, the transistors Tr11 inthe memory cell AM[i,j], the memory cell AM[i,j+1], and the memory cellAM_(ref)[i] are turned on. Thus, in the memory cell AM[i,j], the wiringWD[j] and the node N[i,j] are electrically connected to each other, andthe potential of the node N[i,j] is V_(PR)-V_(X)[i,j]. In the memorycell AM[i,j+1], the wiring WD[j+1] and the node N[i,j+1] areelectrically connected to each other, and the potential of the nodeN[i,j+1] is V_(PR)-V_(X)[i,j+1]. In the memory cell AM_(ref)[i], thewiring WD_(ref) and the node N_(ref)[i] are electrically connected toeach other, and the potential of the node N_(ref)[i] is V_(PR).

A current flowing from the first to second terminal of the transistorTr12 in each of the memory cell AM[i,j], the memory cell AM[i,j+1], andthe memory cell AM_(ref)[i] is considered. The current I₀[i,j] flowingfrom the wiring B[j] to the second terminal through the first terminalof the transistor Tr12 in the memory cell AM[i,j] can be expressed bythe following formula.I ₀[i,j]=k(V _(PR) −V _(X)[i,j]−th)²  (E1)

In the formula, k is a constant determined by the channel length, thechannel width, the mobility, the capacitance of a gate insulating film,and the like of the transistor Tr12. Furthermore, V_(th) is a thresholdvoltage of the transistor Tr12.

At this time, the current flowing from the wiring B[l] to the secondterminal through the first terminal of the transistor Tr12 in the memorycell AM[i,j] is I₀[i,j].

The current I₀[i,j+l] flowing from the wiring B[i+1] to the secondterminal through the first terminal of the transistor Tr12 in the memorycell AM[i,j+1] can be expressed by the following formula.I ₀[i,j+l]=k(V _(PR) −V _(X)[i,j+1]−V _(th))²

At this time, the current flowing from the wiring B[j+1] to the secondterminal through the first terminal of the transistor Tr12 in the memorycell AM[i,j+1] is I₀[i,j+1].

The current I_(ref0)[i] flowing from the wiring B_(ref) to the secondterminal through the first terminal of the transistor Tr12 in the memorycell AM_(ref)[i] can be expressed by the following formula.I _(ref0)[i]=k(V _(PR) −V _(th))²  (E2)

At this time, the current flowing from the wiring B_(ref) to the secondterminal through the first terminal of the transistor Tr12 in the memorycell AM_(ref)[i] is I_(ref0)[i].

Note that since the low-level potential is applied to the gates of thetransistors Tr11 in the memory cell AM[i+1,j], the memory cellAM[i+1,j+1], and the memory cell AM_(ref)[i+1], the transistors Tr11 inthe memory cell AM[i+1,j], the memory cell AM[i+1,j+1], the memory cellAM_(ref)[i+1] are turned off. Thus, the potentials are not retained atthe node N[i+1,j], the node N[i+1,j+1], and the node N_(ref)[i+1].

«Period from Time T02 to Time T03»

During a period from Time T02 to Time T03, the low-level potential isapplied to the wiring WW[i]. At this time, the low-level potential isapplied to the gates of the transistors Tr11 in the memory cell AM[i,j],the memory cell AM[i,j+1], and the memory cell AM_(ref)[i], andaccordingly, the transistors Tr11 in the memory cells AM[i,j],AM[i,j+1], and AM_(ref)[i] are turned off.

The low-level potential has been applied to the wiring WW[i+1]continuously since before Time T02. Thus, the transistors Tr11 in thememory cell AM[i+1,j], the memory cell AM[i+1,j+1], and the memory cellAM_(ref)[i+1] have been kept in an off state since before Time T02.

Since the transistors Tr11 in the memory cell AM[i,j], the memory cellAM[i,j+1], the memory cell AM[i+1,j], the memory cell AM[i+1,j+1], thememory cell AM_(ref)[i], and the memory cell AM_(ref)[i+1] are each inan off state as described above, the potentials at the node N[i,j], thenode N[i,j+1], the node N[i+1,j], the node N[i+1, j+1], the nodeN_(ref)[i], and the node N_(ref)[i+1] are retained in a period from TimeT02 to Time T03.

In particular, when an OS transistor is used as each of the transistorsTr11 in the memory cell AM[i,j], the memory cell AM[i,j+1], the memorycell AM[i+1,j], the memory cell AM[i+1,j+1], the memory cellAM_(ref)[i], and the memory cell AM_(ref)[i+1] as described in thecircuit configuration of the semiconductor device 100, the amount ofleakage current flowing between the sources and the drains of thetransistors Tr11 can be made small, which makes it possible to retainthe potentials at the nodes for a long time.

During the period from Time T02 to Time T03, the ground potential isapplied to the wiring WD[i], the wiring WD[j+1], and the wiringWD_(ref). Since the transistors Tr11 in the memory cell AM[i,j], thememory cell AM[i,j+1], the memory cell AM[i+1,j], the memory cellAM[i+1,j+1], the memory cell AM_(ref)[i], and the memory cellAM_(ref)[i+1] are each in an off state, the potentials retained at thenodes in the memory cell AM[i,j], the memory cell AM[i,j+1], the memorycell AM[i+1,j], the memory cell AM[i+1,j+1], the memory cellAM_(ref)[i], and the memory cell AM_(ref)[i+l] are not rewritten byapplication of potentials from the wiring WD[j], the wiring WD[j+1], andthe wiring WD_(ref).

«Period from Time T03 to Time T04»

During a period from Time T03 to Time T04, the low-level potential isapplied to the wiring WW[i], and a high-level potential is applied tothe wiring WW[i+1]. Furthermore, the potential higher than the groundpotential by V_(PR)-V_(X)[i+1,j] is applied to the wiring WD[j], thepotential higher than the ground potential by V_(PR)-V_(X)[i+1,j+1] isapplied to the wiring WD[j+1], and the potential higher than the groundpotential by V_(PR) is applied to the wiring WD_(ref). Moreover, thereference potential is continuously being applied to the wiring RW[i]and the wiring RW[i+1] continuously since Time T02.

Note that the potential V_(x)[i+1,j] and the potential V_(x)[i+1,j+1]are each a potential corresponding to the first analog data.

In this period, the high-level potential is applied to the gates of thetransistors Tr11 in the memory cell AM[i+1,j], the memory cellAM[i+1,j+1], and the memory cell AM_(ref)[i+1], and accordingly, thetransistors Tr11 in the memory cell AM[i+1,j], the memory cellAM[i+1,j+1], and the memory cell AM_(ref)[i+1] are each brought into anon state. Thus, the node N[i+1,j] in the memory cell AM[i+1,j] iselectrically connected to the wiring WD[j], and the potential of thenode N[i+1,j] becomes V_(PR)-V_(x)[i+1,j]. In the memory cellAM[i+1,j+1], the wiring WD[j+1] and the node N[i+1,j+1] are electricallyconnected to each other, and the potential of the node N[i+1,j+1]becomes V_(PR)-V_(x)[i+1,j+l]. In the memory cell AM_(ref)[i+1], thewiring WD_(ref) and the node N_(ref)[i+1] are electrically connected toeach other, and the potential of the node N_(ref)[i+1] becomes V_(PR).

The current flowing from the first to second terminal of the transistorTr12 in each of the memory cell AM[i+1,j], the memory cell AM[i+1,j+1],and the memory cell AM_(ref)[i+1] is considered. The current I₀[i+1,j]flowing from the wiring B[j] to the second terminal through the firstterminal of the transistor Tr12 in the memory cell AM[i+1,j] can beexpressed by the following formula.I ₀[i+1,j]=k(V _(PR) −V _(X)[i+1,j]−V _(th))²

At this time, the current flowing from the wiring B[j] to the secondterminal through the first terminal of the transistor Tr12 in the memorycell AM[i+1,j] is I₀[i,j]+I₀[i+1,j].

The current I₀[i+1,j+1] flowing from the wiring B[j+1] to the secondterminal through the first terminal of the transistor Tr12 in the memorycell AM[i+1,j+1] can be expressed by the following formula.I ₀[i+1,j+1]=k(V _(PR) −V _(X)[i+1,j+1]−V _(th))²

At this time, the current flowing from the wiring B[j+1] to the secondterminal through the first terminal of the transistor Tr12 in the memorycell AM[i+1,j+1] is I₀[i,j+1]+I₀[i+1,j+1].

The current I_(ref0)[i+1] flowing from the wiring B_(ref) to the secondterminal through the first terminal of the transistor Tr12 in the memorycell AM_(ref)[i+1] can be expressed by the following formula.I _(ref0)[i+1]=k(V _(PR) −V _(th))²

At this time, the current flowing from the output terminal OT_(ref) ofthe reference column output circuit C_(ref) to the wiring B_(ref) isI_(ref0)[i]+I_(ref0)[i+1].

«Period from Time T04 to Time T05»

During a period from Time T04 to Time T05, the potential correspondingto the first analog data is written to the rest of the memory cells AM,and the potential V_(PR) is written to the rest of memory cellsAM_(ref), in a manner similar to that of the operation during the periodfrom Time T01 to Time T02 and that of the operation during the periodfrom Time T03 to Time T04. Thus, the total amount of currents flowing inthe transistors Tr12 in all of the memory cells AM corresponds to theamount of current flowing from the output terminal OT[i] of the columnoutput circuit OUT[i] to the wiring B[j] which is denoted by ΣI₀[i,j] (Erepresents the current obtained by summing over i from 1 to m).

Here, the reference column output circuit C_(ref) is focused on. Thetotal amount of current flowing through the transistors Tr12 in thememory cells AM_(ref)[1] to AM_(ref)[m] flows into the wiring B_(ref) ofthe reference column output circuit C_(ref). In other words, the currentI_(Bref)=ΣI_(ref0)[i] (E represents the current obtained by summing overi from 1 to m) flows into the wiring B_(ref).

Although the current flowing in the wiring IL_(ref) is denoted by I_(CM)in FIG. 6, the current flowing in the wiring IL_(ref) before Time T09 isdenoted by I_(CM0) in this specification.

The current I_(Cref) is outputted from the terminal CT4 of the constantcurrent circuit CI_(ref). Thus, I_(CM0) is determined by setting thepotential of the gate of the transistor Tr7 (potential of the nodeNCM_(ref)) so that the following formula is satisfied.

$\begin{matrix}{{I_{Cref} - I_{{CM}\; 0}} = {\sum\limits_{i}\;{I_{{ref}\; 0}\lbrack i\rbrack}}} & ({E3})\end{matrix}$

Note that since the potential of the gate of the transistor Tr7(potential of the node NCM_(ref)) is used as a reference in the currentmirror circuit CM, the current I_(CM0) also flows in the wirings IL[1]to IL[n] of the column output circuits OUT[1] to OUT[n].

«Period from Time T05 to Time T06»

During a period from Time T05 to Time T06, the wiring ORP is set at thehigh-level potential. At this time, the high-level potential is appliedto the gates of the transistors Tr3 in the column output circuits OUT[1]to OUT[n], so that the transistors Tr3 are brought into an on state.Concurrently, the low-level potential is applied to the first terminalsof the capacitors C1 in the column output circuits OUT[1] to OUT[n], andthus the potentials of the capacitors C1 are initialized. When Time T06starts, the low-level potential is applied to the wiring ORP, so thatthe transistors Tr3 in the column output circuits OUT[1] to OUT[n] arebrought into an off state.

«Period from Time T06 to Time T07»

During a period from Time T06 to Time T07, the wiring ORP is set to thelow-level potential. In the above manner, the low-level potential isapplied to the gates of the transistors Tr3 in the column outputcircuits OUT[1] to OUT[n], so that the transistors Tr3 are brought intoan off state.

«Period from Time T07 to Time T08»

During a period from Time T07 to Time T08, the wiring OSP is set at thehigh-level potential. In the above manner, the high-level potential isapplied to the gates of the transistors Tr2 in the column outputcircuits OUT[1] to OUT[n], so that the transistors Tr2 are brought intoan on state. At this time, the current flows into the first terminals ofthe capacitors C1 from the first terminals of the transistors Tr2through the second terminals of the transistors Tr2, and the potentialsare retained in the capacitors C1. Thus, the potentials of the gates ofthe transistors Tr1 are retained, so that the current corresponding tothe potentials of the gates of the transistors Tr1 flows between thesources and the drains of the transistors Tr1.

When Time T08 starts, the low-level potential is applied to the wiringOSP, so that the transistors Tr2 in the column output circuits OUT[1] toOUT[n] are brought into an off state. The potentials of the gates of thetransistors Tr1 are retained in the capacitors C1, so that even afterTime T08, the same amount of current keeps flowing between the sourcesand the drains of the transistors Tr1.

Here, the column output circuit OUT[j] is focused on. In the columnoutput circuit OUT[j], the current flowing between the source and thedrain of the transistor Tr1 is denoted by I_(CP)[j], and the currentflowing between the source and the drain of the transistor Tr4 of theconstant current circuit CI is denoted by Ic[j]. The current flowingbetween the source and the drain of the transistor Tr5 through thecurrent mirror circuit CM is I_(CM0). On the assumption that the currentis not outputted from the output terminal SPT[i] from Time Ti to TimeT8, the sum of the amounts of current flowing through each of thetransistors Tr12 in the memory cells AM[1] to AM[n] flows in the wiringB[j] of the column output circuit OUT[j]. In other words, the currentΣI₀[i,j] (E represents the current obtained by summing over i from 1 tom) flows in the wiring B[j]. Thus, the above satisfies the followingformula.

$\begin{matrix}{{{I_{C}\lbrack j\rbrack} - I_{{CM}\; 0} - {I_{CP}\lbrack j\rbrack}} = {\sum\limits_{i}\;{I_{0}\left\lbrack {i,j} \right\rbrack}}} & \left( {E\; 4} \right)\end{matrix}$«Period from Time T09 to Time T10»

The operation from Time T09 is described with reference to FIG. 9.During a period from Time T09 to Time T10, a potential higher than thereference potential (denoted by REFP in FIG. 9) by V_(W)[i] is appliedto the wiring RW[i]. At this time, the potential V_(W)[i] is applied tothe second terminals of the capacitors C2 in the memory cells AM[i,1] toAM[i,n] and the memory cell AM_(ref)[i], so that the potentials of thegates of the transistors Tr12 increase.

Note that the potential V_(W)[i] is a potential corresponding to thesecond analog data.

An increase in the potential of the gate of the transistor Tr12corresponds to the potential obtained by multiplying a change inpotential of the wiring RW[i] by a capacitive coupling coefficientdetermined by the memory cell structure. The capacitive couplingcoefficient is calculated on the basis of the capacitance of thecapacitor C2, the gate capacitance of the transistor Tr2, and theparasitic capacitance. In this operation example, to avoid complexity ofdescription, an increase in potential of the wiring RW[i] is equal to anincrease in potential of the gate of the transistor Tr12, whichcorresponds to a case where the capacitive coupling coefficient in eachof the memory cells AM and the memory cell AM_(ref) is set to 1.

When the potential V_(W)[i] is applied to the second terminals of thecapacitors C2 in the memory cell AM[i,j], the memory cell AM[i,j+1], andthe memory cell AM_(ref)[i] on the assumption that the capacitivecoupling coefficient is 1, the potentials of the node N[i,j], the nodeN[i,j+1], and the node N_(ref)[i] each increase by V_(W)[i].

The current flowing from the first to second terminal of the transistorTr12 in each of the memory cell AM[i,j], the memory cell AM[i,j+1], andthe memory cell AM_(ref)[i] is described. The current I[i,j] flowingfrom the wiring B[j] to the second terminal through the first terminalof the transistor Tr12 in the memory cell AM[i,j] can be expressed bythe following formula.I[i,j]=k(V _(PR) −V _(X)[i,j]+V _(W)[i]−V _(th))²  (E5)

In other words, by application of the potential V_(W)[i] to the wiringRW[i], the current flowing from the wiring B[j] to the second terminalthrough the first terminal of the transistor Tr12 in the memory cellAM[i,j] increases by I[i,j]−I₀[i,j] (denoted by ΔI[i,j] in FIG. 9).

The current I[i,j+1] flowing from the wiring B[j+1] to the secondterminal through the first terminal of the transistor Tr12 in the memorycell AM[i,j+1] can be expressed by the following formula.I[i,j+1]=k(V _(PR) −V _(X)[i,j+1+]V _(W)[i]−V _(th))²

In other words, by application of the potential V_(W)[i] to the wiringRW[i], the current flowing from the wiring B[j+1] to the second terminalthrough the first terminal of the transistor Tr12 in the memory cellAM[i,j+1] increases by I[i,j+1]−I₀[i,j+1] (denoted by ΔI[i,j+1] in FIG.9).

Furthermore, the current I_(ref)[i] flowing from the wiring B_(ref) tothe second terminal through the first terminal of the transistor Tr12 inthe memory cell AM_(ref)[i] can be expressed by the following formula.I _(ref)[i]=k(V _(PR) +V _(W)[i]−V _(th))²*  (E6)

In other words, by application of the potential V_(W)[i] to the wiringRW[i], the current flowing from the wiring B_(ref) to the secondterminal through the first terminal of the transistor Tr12 in the memorycell AM_(ref)[i] increases by I_(ref)[i]−1_(ref0)[i] (denoted byΔI_(ref)[i] in FIG. 9).

Here, the reference column output circuit C_(ref) is focused on. Thetotal amount of current flowing through the transistors Tr12 in thememory cells AM_(ref)[1] to AM_(ref)[m] flows into the wiring B_(ref) ofthe reference column output circuit C_(ref). In other words, the currentI_(Bref)=ΣI_(ref0)[i] flows into the wiring B_(ref).

The current I_(Cref) is outputted from the terminal CT4 in the constantcurrent circuit CI_(ref). Thus, I_(CM) is determined by setting thepotential of the gate of the transistor Tr7 (potential of the nodeNCM_(ref)) so that the following formula is satisfied.

$\begin{matrix}{{I_{Cref} - I_{CM}} = {\sum\limits_{i}\;{I_{ref}\lbrack i\rbrack}}} & \left( {E\; 7} \right)\end{matrix}$

Here, the current ΔI_(B)[j] outputted from the wiring B[j] is focusedon. From Time T08 to Time T09, Formula (E4) is satisfied, and thecurrent ΔI_(B)[i] is not outputted from the wiring B[j].

During the period from Time T09 to Time T10, a potential higher than thereference potential by V_(W)[i] is applied to the wiring RW[i], and thecurrent flowing between the source and the drain of the transistor Tr12in the memory cell AM[i,j] changes. Then, the current ΔI_(B)[i] isoutputted from the output terminal SPT[j] that is electrically connectedto the wiring B[j]. Specifically, in the column output circuit OUT[j],the current Ic[i] is outputted from the terminal CT2 of the constantcurrent circuit CI, the current I_(CM) flows between the source and thedrain of the transistor Tr5, and the current I_(CP)[i] flows between thesource and the drain of the transistor Tr1. Thus, the current ΔI_(B)[i]can be expressed by the following formula using ΣI[i,j] where thecurrent flowing between the source and the drain of the transistor Tr12in the memory cell AM[i,j] is calculated by summing over i from 1 to m.

$\begin{matrix}{{\Delta\;{I_{B}\lbrack j\rbrack}} = {\left( {{I_{C}\lbrack j\rbrack} - I_{CM} - {I_{CP}\lbrack j\rbrack}} \right) - {\sum\limits_{i}\;{I\left\lbrack {i,j} \right\rbrack}}}} & ({E8})\end{matrix}$

Formulae (E1) to (E7) are applied to Formula (E8), so that the followingformula can be obtained.

$\begin{matrix}{{\Delta\;{I_{B}\lbrack j\rbrack}} = {2\; k{\sum\limits_{i}\left( {{V_{X}\left\lbrack {i,j} \right\rbrack}{V_{W}\lbrack i\rbrack}} \right)}}} & ({E9})\end{matrix}$

According to Formula (E9), the current ΔI_(B)[i] is a valuecorresponding to the sum of products of the potential V_(X)[i,j] that isthe first analog data and the potential V_(W)[i] that is the secondanalog data. Thus, when the current ΔI_(B)[i] is calculated, the valueof the sum of products of the first analog data and the second analogdata can be obtained.

During the period from Time T09 to Time T10, when all of the wiringsRW[1] to RW[m] except the wiring RW[i] are set to have a referencepotential, the relation, V_(W)[g]=0 (g is an integer that is greaterthan or equal to 1 and less than or equal to m and not i), is satisfied.Thus, according to Formula (E9), ΔI_(B)[i]=2 kV_(X)[i,j]V_(W)[i] isoutputted. In other words, the data corresponding to the product of thefirst analog data stored in the memory cell AM[i,j] and the secondanalog data corresponding to a selection signal supplied to the wiringRW[i] is outputted from the output terminal SPT[j] that is electricallyconnected to the wiring B[j].

Furthermore, a differential current outputted from the output terminalSPT[j+1] that is electrically connected to the wiring B[j+1] isexpressed as ΔI_(B)[i+1]=2 kV_(X)[i,j+1]V_(W)[i]. The data correspondingto the product of the first analog data stored in the memory cellAM[i,j+1] and the second analog data corresponding to a selection signalsupplied to the wiring RW[i] is outputted from the output terminalSPT[j+1] that is electrically connected to the wiring B[i+1].

«Period from Time T10 to Time T11»

During a period from Time T10 to Time T11, the ground potential isapplied to the wiring RW[i]. The ground potential is applied to thesecond terminals of the capacitors C2 in the memory cells AM[i,1] toAM[i,n] and the memory cell AM_(ref)[i]. Thus, the potentials of thenodes N[i,1] to N[i,n] and the node N_(ref)[i] return to the potentialsduring the period from Time T08 to Time T09.

«Period from Time T11 to Time T12»

During a period from Time T11 to Time T12, the wirings RW[1] to RW[m]except the wiring RW[i+1] are set to have the reference potential, and apotential higher than the reference potential by V_(W)[i+l] is appliedto the wiring RW[i+1]. At this time, as in the operation during theperiod from Time T09 to Time T10, the potential V_(W)[i+1] is applied tothe second terminals of the capacitors C2 in the memory cells AM[i+1,1]to AM[i+1,n] and the memory cell AM_(ref)[i+l], so that the potentialsof the gates of the transistors Tr12 increase.

The potential V_(W)[i+l] corresponds to the second analog data.

As described above, the capacitive coupling coefficients of the memorycells AM and the memory cell AM_(ref) are each 1. When the potentialV_(W)[i+1] is applied to the second terminals of the capacitors C2 inthe memory cell AM[i+1,j], the memory cell AM[i+1,j+l], and the memorycell AM_(ref)[i+l], the potentials of the node N[i+1,j], the nodeN[i+1,j+1], and the node N_(ref)[i+l] each increase by V_(W)[i+l].

When the potentials of the node N[i+1,j], the node N[i+1,j+1], and thenode N_(ref)[i+1] increase by V_(W)[i+1], the amount of current flowingin each of the transistors Tr12 in the memory cell AM[i+1,j], the memorycell AM[i+1,j+1], and the memory cell AM_(ref)[i+1] increases. When thecurrent flowing in the transistor Tr12 in the memory cell AM[i+1,j] isdenoted by I[i+1,j], the current flowing from the output terminal OT[j]of the column output circuit OUT[j] to the wiring B[j] increases byI[i+1,j]−I₀[i+1,j] (denoted by ΔI[i+1,j] in FIG. 9). When the currentflowing in the transistor Tr12 in the memory cell AM[i+1,j+1] is denotedby I[i+1,j+1], the current flowing from the output terminal OT[i+1] ofthe column output circuit OUT[j+1] to the wiring B[j+1] increases byI[i+1,j+1]−1₀[i+1,j+1] (denoted by ΔI[i+1,j+] in FIG. 9). When thecurrent flowing in the transistor Tr12 in the memory cell AM_(ref)[i+l]is denoted by I_(ref)[i+l], the current flowing from the output terminalOT_(ref) of the reference column output circuit C_(ref) to the wiringB_(ref) increases by I_(ref)[i+1]−1_(ref0)[i+l] (denoted byΔI_(ref)[i+l] in FIG. 9).

The operation during the period from Time T11 to Time T12 can be similarto the operation during the period from Time T09 to Time T10. Thus, whenFormula (E9) is applied to the operation during the period from Time T11to Time T12, the differential current outputted from the wiring B[j] isexpressed as ΔI_(B)[j]=2 kV_(x)[i+1,j] V_(W)[i+1]. In other words, thedata corresponding to the product of the first analog data stored in thememory cell AM[i+1,j] and the second analog data corresponding to aselection signal applied to the wiring RW[i+1] is outputted from theoutput terminal SPT[i] that is electrically connected to the wiringB[j].

Furthermore, the differential current outputted from the wiring B[i+1]is expressed as ΔI_(B)[U+1]=2 kV_(x)[i+1,j+1] V_(W)[i+1]. The datacorresponding to the product of the first analog data stored in thememory cell AM[i+1,j+1] and the second analog data corresponding to aselection signal applied to the wiring RW[i+1] is outputted from theoutput terminal SPT[j+1] that is electrically connected to the wiringB[j+1].

«Period from Time T12 to Time T13»

During a period from Time T12 to Time T13, the ground potential isapplied to the wiring RW[i+1]. In this period, the ground potential isapplied to the second terminals of the capacitors C2 in the memory cellsAM[i+1,1] to AM[i+1,n] and the memory cell AM_(ref)[i+1], and thepotentials of nodes N[i+1,1] to N[i+1,n] and the node N_(ref)[i+1]return to the potentials in the period from Time T10 to Time T11.

«Period from Time T13 to Time T14»

During a period from Time T13 to Time T14, the wirings RW[1] to RW[m]except the wiring RW[i] and the wiring RW[i+1] are set to have thereference potential, a potential higher than the reference potential byV_(W2)[i] is applied to the wiring RW[i], and a potential lower than thereference potential by V_(W2)[i+1] is applied to the wiring RW[i+1]. Atthis time, as in the operation during the period from Time T09 to TimeT10, the potential V_(W2)[i] is applied to the second terminals of thecapacitors C2 in the memory cells AM[i,1] to AM[i,n] and the memory cellAM_(ref)[i], so that potentials of the gates of the transistors Tr12 inthe memory cells AM[i,1] to AM[i,n] and the memory cell AM_(ref)[i]increase. Concurrently, the potential −V_(W2)[i+1] is applied to thesecond terminals of the capacitors C2 in the memory cells AM[i+1,1] toAM[i+1,n] and the memory cell AM_(ref)[i+1], so that the potentials ofthe gates of the transistors Tr12 in the memory cells AM[i+1,1] toAM[i+1,n] and the memory cell AM_(ref)[i+1] decrease.

The potential V_(W2)[i] and the potential V_(W2)[i+1] are potentialseach corresponding to the second analog data.

Note that the capacitive coupling coefficients of the memory cell AM andthe memory cell AM_(ref) are each 1. When the potential V_(W2)[i] isapplied to the second terminals of the capacitors C2 in the memory cellAM[i,j], the memory cell AM[i,j+1], and the memory cell AM_(ref)[i], thepotentials of the node N[i,j], the node N[i,j+1], and the nodeN_(ref)[i] each increase by V_(W2)[i]. When the potential −V_(W2)[i+1]is applied to the second terminals of the capacitors C2 in the memorycell AM[i+1,j], the memory cell AM[i+1,j+1], and the memory cellAM_(ref)[i+1], the potentials of the node N[i+1,j], the node N[i+1,j+l], and the node N_(ref)[i+1] each decrease by V_(W2)[i+1].

When each of the potentials of the node N[i,j], the node N[i,j+1], andthe node N_(ref)[i] increases by V_(W2)[i], the amount of currentflowing in each of the transistors Tr12 in the memory cell AM[i,j], thememory cell AM[i,j+1], and the memory cell AM_(ref)[i] increases. Here,the current flowing in the transistor Tr12 in the memory cell AM[i,j] isdenoted by I[i,j], the current flowing in the transistor Tr12 in thememory cell AM[i,j+1] is denoted by I[i,j+1], and the current flowing inthe transistor Tr12 in the memory cell AM_(ref)[i] is denoted byI_(ref)[i].

When the potentials of the node N[i+1,j], the node N[i+1,j+1], and thenode N_(ref)[i+1] each decrease by V_(W2)[i+1], the amount of currentflowing in each of the transistors Tr12 in the memory cell AM[i+1,j],the memory cell AM[i+1,j+1], and the memory cell AM_(ref)[i+1]decreases. Here, the current flowing in the transistor Tr12 in thememory cell AM[i+1,j] is denoted by I₂[i,j], the current flowing in thetransistor Tr12 in the memory cell AM[i+1,j+1] is denoted by I₂[i,j+1],and the current flowing in the transistor Tr12 in the memory cellAM_(ref)[i+1] is denoted by I_(2ref)[i+l].

At this time, the current flowing from the output terminal OT[j] of thecolumn output circuit OUT[j] to the wiring B[j] increases by(I₂[i,j]−1₀[i,j])+(I₂[i+1,j]−1₀[i+1,j]) (denoted by ΔI[i] in FIG. 9).The current flowing from the output terminal OT[j+1] of the columnoutput circuit OUT[j+1] to the wiring B[j+1] increases by(I₂[i,j+1]−1₀[i,j+1])+(I₂[i+1,j+1]−1₀[i+1,j+1]) (denoted by ΔI[j+1] inFIG. 9, which is a negative current). The current flowing from theoutput terminal OT_(ref) of the reference column output circuit C_(ref)to the wiring B_(ref) increases by(I_(2ref)[i,j]−1_(ref0)[i,j])+(I_(2ref)[i+1,j]−1_(ref0)[i+1,j]) (denotedby ΔI_(Bref) in FIG. 9).

The operation during the period from Time T13 to Time T14 can be similarto that operation during the period from Time T09 to Time T10. WhenFormula (E9) is applied to the operation during the period from Time T13to Time T14, the differential current outputted from the wiring B[j] isexpressed as ΔI_(B)[j]=2k{V_(X)[i,j]V_(W2)[i]−V_(x)[i+1,j]V_(W2)[i+1]}.Thus, the data corresponding to the sum of products of the first analogdata stored in each of the memory cell AM[i,j] and the memory cellAM[i+1,j] and the second analog data corresponding to a selection signalapplied to each of the wiring RW[i] and the wiring RW[i+1] is outputtedfrom the output terminal SPT[j] that is electrically connected from thewiring B[j].

The differential current outputted from the wiring B[j+1] is expressedas ΔIB[j+1]=2k{V_(X)[i,j+l]V_(W2)[i]−V_(x)[i+1,j+1]V_(W2)[i+1]}. Thedata corresponding to the product of the first analog data stored ineach of the memory cell AM[i,j+1] and the memory cell AM[i+1,j+1] andthe second analog data corresponding to a selection signal applied toeach of the wiring RW[i] and the wiring RW[i+1] is outputted from theoutput terminal SPT[j+1] that is electrically connected to the wiringB[i+1].

«After Time T14»

From and after Time T14, the ground potential is applied to the wiringRW[i] and the wiring RW[i+1]. At this time, the ground potential isapplied to the second terminals of the capacitors C2 in the memory cellsAM[i,1] to AM[i,n], the memory cells AM[i,1+1] to AM[i+1,n], the memorycell AM_(ref)[i], and the memory cell AM_(ref)[i+1]. Thus, thepotentials of the nodes N[i,1] to N[i,n], the nodes N[i+1,1] toN[i+1,n], the node N_(ref)[i], and the node N_(ref)[i+1] return to thepotentials in the period from Time T12 to Time T13.

With the circuit configuration of FIG. 1, a plurality of product-sumoperations can be executed concurrently. Thus, a semiconductor deviceenabling high-speed product-sum operation can be provided.

Note that this embodiment can be combined with any of the otherembodiments in this specification as appropriate.

Embodiment 2

In this embodiment, application examples of the semiconductor devicedescribed in the above embodiment to electronic devices will bedescribed with reference to FIGS. 10A to 10E.

<Electronic Component>

FIG. 10A illustrates an example where the semiconductor device describedin the foregoing embodiment is used as a memory device in an electroniccomponent. Note that the electronic component is also referred to as asemiconductor package or an IC package. This electronic component has aplurality of standards and names depending on a terminal extractiondirection and a terminal shape. Thus, examples of the electroniccomponent are described in this embodiment.

A semiconductor device including the transistors described in Embodiment1 is completed through an assembly process (post-process) of integratingdetachable components on a printed circuit board.

The post-process can be completed through steps in FIG. 10A.Specifically, after an element substrate obtained in the proceedingprocess is completed (Step STP1), a rear surface of the substrate isground (Step STP2). The substrate is thinned in this step to reducewarpage or the like of the substrate in the proceeding process and toreduce the size of the component itself.

After the rear surface of the substrate is ground, a dicing step isperformed to divide the substrate into a plurality of chips (Step STP3).Then, the divided chips are separately picked up, placed on a leadframe, and bonded thereto in a die bonding step (Step STP4). In this diebonding step, the chip is bonded to the lead frame by an appropriatemethod depending on a product, for example, bonding with a resin or atape. Note that in the die bonding step, the chip may be mounted on aninterposer to be bonded.

Note that in this embodiment, when an element is formed on a surface ofa substrate, the other surface is referred to as a rear surface (asurface on which the element is not formed).

Next, wire bonding for electrically connecting a lead of the lead frameand an electrode on a chip through a metal wire is performed (StepSTP5). A silver line or a gold line can be used as the metal fine line.Ball bonding or wedge bonding can be used as the wire bonding.

A wire-bonded chip is subjected to a molding step of sealing the chipwith an epoxy resin or the like (Step STP6). With the molding step, theinside of the electronic component is filled with a resin, therebyreducing damage to the circuit portion and the wire embedded in thecomponent caused by external mechanical force as well as reducingdeterioration of characteristics due to moisture or dust.

Next, plate processing is performed on the lead of the lead frame. Then,the lead is cut and processed into a predetermined shape (Step STP7).This plate processing prevents rust of the lead and facilitatessoldering at the time of mounting the chip on a printed circuit board ina later step.

Next, printing (marking) is performed on a surface of the package (StepSTP8). After a final testing step (Step STP9), the electronic componentis completed (Step STP10).

The above electronic component can include the semiconductor devicedescribed in the above embodiment. Thus, a highly reliable electroniccomponent can be obtained.

FIG. 10B is a schematic perspective diagram of the completed electroniccomponent. FIG. 10B is a schematic perspective diagram of a quad flatpackage (QFP) as an example of the electronic component. An electroniccomponent 4700 in FIG. 10B includes a lead 4701 and a circuit portion4703. The electronic component 4700 in FIG. 10B is mounted on a printedcircuit board 4702, for example. A plurality of electronic components4700 which are combined and electrically connected to each other overthe printed circuit board 4702 can be mounted on an electronic device. Acompleted circuit board 4704 is provided in an electronic device or thelike.

One embodiment of the present invention is not limited to the shape ofthe electronic component 4700, and the element substrate fabricated inStep STP1 is included. In addition, the element substrate of oneembodiment of the present invention includes an element substrate thathas been subjected to Step STP2 where the rear surface of the substrateis ground. In addition, the element substrate of one embodiment of thepresent invention includes an element substrate that has been subjectedto Step STP3 where the dicing step is performed. For example, asemiconductor wafer 4800 shown in FIG. 10C or the like corresponds tothe element substrate. In the semiconductor wafer 4800, a plurality ofcircuit portions 4802 are formed on a top surface of a wafer 4801. Apart without the circuit portions 4802 on the top surface of the wafer4801 is a spacing 4803 that is a region for dicing.

The dicing is carried out along scribe lines SCL1 and scribe lines SCL2(referred to as dicing line or cutting line in some cases) indicated bydashed-dotted lines. For performing the dicing step easily, the spacing4803 is preferably provided such that a plurality of scribe lines SCL1are parallel to each other, a plurality of scribe lines SCL2 areparallel to each other, and the scribe lines SCL1 and the scribe linesSCL2 are intersected perpendicularly with each other.

With the dicing step, a chip 4800 a as shown in FIG. 10D can be cut outfrom the semiconductor wafer 4800. The chip 4800 a includes a wafer 4801a, the circuit portion 4802, and a spacing 4803 a. Note that it ispreferable to provide the spacing 4803 a to be made as small aspossible. In this case, it is preferable that the width of the spacing4803 between adjacent circuit portions 4802 be substantially the same asa length of margin for cutting of the scribe line SCL1 or the scribeline SCL2.

The shape of the element substrate of one embodiment of the presentinvention is not limited to the shape of the semiconductor wafer 4800shown in FIG. 10C. For example, a rectangular semiconductor wafer 4810shown in FIG. 10E can be employed. The shape of the element substratecan be changed as appropriate, depending on a process for fabricating anelement and an apparatus for fabricating an element.

Note that this embodiment can be combined with any of the otherembodiments in this specification as appropriate.

Embodiment 3

In this embodiment, application examples of the semiconductor devicedescribed in Embodiment 1 will be described.

Since the semiconductor device 100 functions as a product-sum operationcircuit, the semiconductor device 100 can be used as one of componentsof an artificial neural network in some cases. The artificial neuralnetwork is an information processing system modeled on a biologicalneural network and enables the characteristics of a brain function to beexpressed on a computer.

The artificial neural network has a configuration where units resemblingneurons are connected to each other through units resembling synapses.It is considered that when the connection strength is changed, theartificial neural network learns about a variety of input patterns andcomes to enable pattern recognition, associative storage, and datamining to be executed at high speed. In particular, a novel electronicdevice utilizing pattern recognition of sound, voice, music, images,videos, or the like can be achieved in some cases with the artificialneural network.

In the semiconductor device described in Embodiment 1, the first analogdata serves as weight coefficients, and the second analog datacorresponds to neuron outputs, whereby the weighting calculation ofneuron outputs can be conducted concurrently. Thus, data correspondingto results of the weighting calculation, that is, synapse inputs can beobtained as the output signals.

<Electronic Device>

Here, electronic devices or systems utilizing the artificial neuralnetwork are described.

FIG. 11A illustrates a tablet information terminal, which includes ahousing 5221, a display portion 5222, operation buttons 5223, and aspeaker 5224. A display device with a position input function may beused for a display portion 5222. Note that the position input functioncan be added by provision of a touch panel in a display device.Alternatively, the position input function can be added by providing aphotoelectric conversion element called a photosensor in a pixel area ofa display device. As the operation buttons 5223, any one of a powerswitch for starting the information terminal, a button for operating anapplication of the information terminal, a volume control button, aswitch for turning on or off the display portion 5222, and the like canbe provided. Although the number of the operation buttons 5223 is fourin the information terminal illustrated in FIG. 11A, the number andposition of operation buttons included in the information terminal isnot limited to this example. Although not illustrated, the informationterminal illustrated in FIG. 11A may be provided with a camera. Althoughnot illustrated, the information terminal illustrated in FIG. 11A mayinclude a light-emitting device for use as a flashlight or a lightingdevice. Although not illustrated, the information terminal illustratedin FIG. 11A may include a sensor (which measures force, displacement,position, speed, acceleration, angular velocity, rotational frequency,distance, light, liquid, magnetism, temperature, a chemical substance, asound, time, hardness, electric field, current, voltage, electric power,radiation, flow rate, humidity, gradient, oscillation, smell, infraredrays, or the like) inside the housing 5221. In particular, when asensing device including a sensor for detecting inclination, such as agyroscope or acceleration sensor is provided, display on the screen ofthe display portion 5222 can be automatically changed in accordance withthe orientation of the information terminal illustrated in FIG. 11A bydetermining the orientation of the information terminal (the orientationof the information terminal with respect to the vertical direction). Inthe case where a device for obtaining biological information offingerprints, veins, iris, voice prints, or the like is provided, aninformation terminal having a function of biometric authentication canbe obtained. In particular, the artificial neural network is used forthe biometric authentication, whereby a high-accuracy authenticationsystem can be formed in some cases. Furthermore, the application of theartificial neural network in the information terminal is not limitedonly to the authentication system. For example, in the informationterminal utilizing the artificial neural network, speech interpretationcan be performed in some cases. With the speech interpretation function,the information terminal can have a function of operating the tabletinformation terminal by speech recognition, a function of interpreting aspeech or a conversation and creating a summary of the speech or theconversation, and the like. Thus, the tablet information terminal can beused as a text for learning foreign languages, for example.Alternatively, the tablet information terminal can be utilized to createmeeting minutes or the like, for example.

Note that without limitation to the tablet information terminalillustrated in FIG. 11A, one embodiment of the present invention may bea mode of a mobile phone (smartphone) as illustrated in FIG. 11B, whichincludes the small-sized information terminal illustrated in FIG. 11Aand has a telephone function. The mobile phone illustrated in FIG. 11Bincludes a housing 5501, a display portion 5502, a microphone 5503, aspeaker 5504, and operation buttons 5505. When a device for obtainingbiological information of fingerprints, veins, iris, voice prints, orthe like is provided for the mobile phone like the information terminalillustrated in FIG. 11A, an information terminal having a biometricauthentication function utilizing the artificial neural network can beachieved in some cases. Furthermore, like the information terminalillustrated in FIG. 11A, the mobile phone may have a speechinterpretation function utilizing the artificial neural network.

FIG. 11C illustrates a video camera including a first housing 5801, asecond housing 5802, a display portion 5803, operation keys 5804, a lens5805, a joint 5806, and the like. The semiconductor device of oneembodiment of the present invention can be used for a variety ofintegrated circuits included in video cameras. The operation keys 5804and the lens 5805 are provided in the first housing 5801, and thedisplay portion 5803 is provided in the second housing 5802. The firsthousing 5801 and the second housing 5802 are connected to each otherwith the joint 5806, and the angle between the first housing 5801 andthe second housing 5802 can be changed with the joint 5806. Imagesdisplayed on the display portion 5803 may be switched in accordance withthe angle at the joint 5806 between the first housing 5801 and thesecond housing 5802.

When images taken by a video camera are recorded, the images need to beencoded in accordance with a data recording format. In encoding of theimages, the pattern recognition of the artificial neural network isutilized to calculate a difference between the image data taken at theprevious time and image data taken this time, so that the data can becompressed.

FIG. 11D illustrates a palm print authentication device, which includesa housing 5431, a display portion 5432, a palm print reading portion5433, and a wiring 5434. Also, FIG. 11D illustrates a situation where apalm print of a hand 5435 is obtained. Without limitation to the palmprint authentication device, one embodiment of the present invention maybe a device for obtaining biological information of fingerprints, veins,faces, iris, voice prints, genes, or the like.

FIG. 12 schematically illustrates data transmission in the broadcastsystem. FIG. 12 illustrates a path in which a radio wave (a broadcastsignal) transmitted from a broadcast station 6561 is delivered to atelevision receiver (TV) 6560 of every household. The TV 6560 isprovided with a receiver and a display device. As examples of anartificial satellite 6562, a communication satellite (CS) and abroadcast satellite (BS) can be given. As examples of an antenna 6564, aBS⋅110° CS antenna and a CS antenna can be given. Examples of theantenna 6565 include an ultra-high frequency (UHF) antenna.

Radio waves 6566A and 6566B are broadcast signals for a satellitebroadcast. When the artificial satellite 6562 receives the radio wave6566A, the artificial satellite 6562 transmits the radio wave 6566B tothe ground. The antenna 6564 of every household receives the radio wave6566B, and a satellite TV broadcast can be watched on the TV 6560.Alternatively, the radio wave 6566B is received by an antenna of anotherbroadcast station, and a receiver in the broadcast station processes theradio wave 6566B into a signal that can be transmitted to an opticalcable. The broadcast station transmits the broadcast signal to the TV6560 of every household using an optical cable network. Radio waves6567A and 6567B are broadcast signals for a terrestrial broadcast. Aradio wave tower 6563 amplifies the received radio wave 6567A andtransmits it as the radio wave 6567B. A terrestrial TV broadcast can bewatched on the TV 6560 of every household when the antenna 6565 receivesthe radio wave 6567B.

Before the radio wave 6566A or the radio wave 6567A is transmitted fromthe broadcast station 6561, the encoding processing of the image data isconducted by an electronic device in the broadcast station 6561. At thistime, the pattern recognition of the artificial neural network isutilized to calculate a difference between the image data taken at theprevious time and the image data taken this time, so that the data canbe compressed.

A video distribution system of this embodiment is not limited to asystem for a TV broadcast. Video data to be distributed may be eithermoving image data or still image data.

For example, video data of a camera may be distributed via a high-speedIP network. The distribution system of the video data can be used in,for example, the medical field for remote diagnosis and remotetreatment. Images with higher resolution are demanded for accurate imagediagnoses and medical care; images with high resolution, such as 8K, 4K,and 2K images, are thus demanded for images for medical use. FIG. 13schematically illustrates an emergency medical system that utilizes thevideo data distribution system.

A high-speed network 6605 performs communication between an emergencytransportation vehicle (an ambulance) 6600 and a medical institution6601 and between the medical institution 6601 and a medical institution6602. The ambulance 6600 is equipped with a camera 6610, an encoder6611, and a communication device 6612.

A patient taken to the medical institution 6601 is photographed with thecamera 6610. Video data 6615 obtained by the camera 6610 may betransmitted without compression by the communication device 6612. Thisenables the transmission of the high-resolution video data 6615 to themedical institution 6601 with little delay. In the case where thehigh-speed network 6605 cannot be used for the communication between theambulance 6600 and the medical institution 6601, the video data can beencoded with the encoder 6611 and encoded video data 6616 can betransmitted. At this time, the pattern recognition of the artificialneural network is utilized, whereby the video data can be compressed ina manner similar to that of the above television system.

In the medical institution 6601, a communication device 6620 receivesthe video data transmitted from the ambulance 6600. When the receivedvideo data is uncompressed data, the data is transmitted via thecommunication device 6620 and displayed on a display device 6623. Whenthe video data is compressed data, the data is decompressed with adecoder 6621 and then transmitted to a server 6622 and the displaydevice 6623. Judging from the image on the display device 6623, doctorsgive instructions to crews of the ambulance 6600 or staff members in themedical institution 6601 who treat the patient. The doctors can checkthe condition of the patient in detail in the medical institution 6601while the patient is taken by the ambulance because the distributionsystem in FIG. 13 can transmit a high-definition image. Therefore, thedoctors can instruct the ambulance crews or the staff membersappropriately in a short time, resulting in improvement of a lifesavingrate of patients.

The communication of video data between the medical institution 6601 andthe medical institution 6602 can be performed in the same way. A medicalimage obtained from an image diagnostic device (such as CT or MRI) ofthe medical institution 6601 can be transmitted to the medicalinstitution 6602. Here, the ambulance 6600 is given as an example of themeans to transport patients; however, an aircraft such as a helicopteror a vessel may be used.

The electronic device or system described in this embodiment can becombined with any other electronic devices or systems as appropriate.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 4

In this embodiment, an input/output device that can be provided for thetablet terminal in FIG. 11A or the mobile phone in FIG. 11B described inEmbodiment 3 will be described.

FIGS. 14A, 14B-1, 14B-2, and 14C illustrate a structure of a touch panel700TP1 that can be used for an input/output device. FIG. 14A is a topview of the touch panel. FIG. 14B-1 is a schematic view illustratingpart of an input portion of the touch panel. FIG. 14B-2 is a schematicview illustrating part of the structure of FIG. 14B-1. FIG. 14C is aschematic view illustrating part of a display portion included in thetouch panel.

FIG. 15A is a bottom view illustrating part of the structure of a pixelin the touch panel in FIG. 14C. FIG. 15B is a bottom view illustratingpart of the structure in FIG. 15A in which some components are omitted.

FIGS. 16A and 16B and FIGS. 17A and 17B are cross-sectional viewsillustrating the structure of the touch panel. FIG. 16A is across-sectional view taken along bold lines Z1-Z2, Z3-Z4, and Z5-Z6 inFIG. 14A. FIG. 16B illustrates part of FIG. 16A.

FIG. 17A is a cross-sectional view taken along bold lines Z7-Z8, Z9-Z10,and Z11-Z12 in FIG. 14A. FIG. 17B illustrates part of FIG. 17A.

FIGS. 18A to 18C are schematic views each illustrating the shape of areflective film that can be used in a pixel of the touch panel.

FIG. 19 is a block diagram illustrating the structure of the inputportion of the touch panel.

FIG. 20 is a circuit diagram illustrating a configuration of a pixelcircuit included in the input/output device.

<Structure Example of Input/Output Device>

The input/output device described in this embodiment includes the touchpanel 700TP1 (see FIG. 14A). Note that the touch panel includes thedisplay portion and the input portion.

«Structure Example of Display Portion»

The display portion includes a display panel, and the display panelincludes a pixel 702(i,j).

The pixel 702(i,j) includes a second conductive film, a first conductivefilm, a second insulating film 501C, and a first display element750(i,j) (see FIG. 17A).

The second conductive film is electrically connected to a pixel circuit530(i,j). For example, a conductive film 512B which functions as asource electrode or a drain electrode of a transistor used as a switchSW1 of the pixel circuit 530(i,j) can be used as the second conductivefilm (see FIG. 17A and FIG. 20).

The first conductive film includes a region overlapping with the secondconductive film. For example, the first conductive film can be used fora first electrode 751(i,j) of the first display element 750(i,j).

The second insulating film 501C includes a region sandwiched between thesecond conductive film and the first conductive film. The secondinsulating film 501C has an opening 591A in the region sandwichedbetween the first conductive film and the second conductive film.Furthermore, the second insulating film 501C includes a regionsandwiched between a first insulating film 501A and a conductive film511B. Moreover, the second insulating film 501C has an opening 591B inthe region sandwiched between the first insulating film 501A and theconductive film 511B. The second insulating film 501C has an opening591C in a region sandwiched between the first insulating film 501A and aconductive film 511C (see FIG. 16A and FIG. 17A).

The first conductive film is electrically connected to the secondconductive film in the opening 591A. For example, the first electrode751(i,j) is electrically connected to the conductive film 512B. Thefirst conductive film electrically connected to the second conductivefilm through the opening 591A provided in the second insulating film501C can be referred to as a through electrode.

The first display element 750(i,j) is electrically connected to thefirst conductive film.

The first display element 750(i,j) includes a reflective film and has afunction of controlling the intensity of light reflected by thereflective film. For example, the first conductive film, the firstelectrode 751(i,j), or the like can be used as the reflective film ofthe first display element 750(i,j). Similarly, the first conductivefilm, a first electrode 751(i,j+1), or the like can be used as areflective film of a first display element 750(i,j+1), and the firstconductive film, a first electrode 751(i,j+2), or the like can be usedas a reflective film of a first display element 750(i,j+2) (see FIG.18A). Note that also in FIG. 18B described later, the first electrode751(i,j), a first electrode 751(i+1,j), and a first electrode 751(i+2,j)are illustrated as the reflective film.

The second display element 550(i,j) has a function of emitting lighttoward the second insulating film 501C (see FIG. 16A).

The reflective film has a shape including a region that does not blocklight emitted from the second display element 550(i,j).

The reflective film included in the pixel 702(i,j) of the display paneldescribed in this embodiment includes one or a plurality of openings751H (see FIGS. 18A to 18C).

The second display element 550(i,j) has a function of emitting lighttoward the opening 751H. Note that the opening 751H transmits lightemitted from the second display element 550(i,j).

The opening 751H of the pixel 702(i,j+1), which is adjacent to the pixel702(i,j), is not provided on a line that extends in the row direction(the direction indicated by an arrow Ro1 in the drawing) through theopening 751H of the pixel 702(i,j) (see FIG. 18A). Alternatively, forexample, the opening 751H of the pixel 702(i+1,j), which is adjacent tothe pixel 702(i,j), is not provided on a line that extends in the columndirection (the direction indicated by an arrow Co1 in the drawing)through the opening 751H of the pixel 702(i,j) (see FIG. 18B).

For example, the opening 751H of the pixel 702(i,j+2) is provided on aline that extends in the row direction through the opening 751H of thepixel 702(i,j) (see FIG. 18A). In addition, the opening 751H of thepixel 702(i,j+1) is provided on a line that is perpendicular to theabove-mentioned line between the opening 751H of the pixel 702(i,j) andthe opening 751H of the pixel 702(i,j+2).

Alternatively, for example, the opening 751H of the pixel 702(i+2,j) isprovided on a line that extends in the column direction through theopening 751H of the pixel 702(i,j) (see FIG. 18B). In addition, forexample, the opening 751H of the pixel 702(i+1,j) is provided on a linethat is perpendicular to the above-mentioned line between the opening751H of the pixel 702(i,j) and the opening 751H of the pixel 702(i+2,j).

Thus, a third display element that displays a color different from thatdisplayed by the second display element can be provided easily near thesecond display element. As a result, a display panel with highconvenience or high reliability can be provided.

For example, the reflective film can be formed using a material having ashape in which an end portion is cut off so as to form a region 751Ethat does not block light emitted from the second display element550(i,j) (see FIG. 18C). Specifically, the first electrode 751(i,j)whose end portion is cut off so as to be shorter in the column direction(the direction indicated by the arrow Co1 in the drawing) can be used asthe reflective film. Note that the first electrode 751(i,j+1) isillustrated in FIG. 18C in addition to the first electrode 751(i,j).

Thus, the first display element and the second display element thatdisplays an image using a method different from that of the firstdisplay element can be driven using pixel circuits that can be formed inthe same process. Specifically, a reflective display element is used asthe first display element, whereby the power consumption can be reduced.In addition, an image with high contrast can be favorably displayed inan environment with bright external light. In addition, the seconddisplay element that emits light is used, whereby an image can befavorably displayed in a dark environment. Furthermore, using the secondinsulating film, impurity diffusion between the first display elementand the second display element or between the first display element andthe pixel circuit can be suppressed. Moreover, part of light emittedfrom the second display element to which a voltage controlled on thebasis of the control data is supplied is not blocked by the reflectivefilm included in the first display element. As a result, a displaydevice with high convenience or high reliability can be provided.

The second display element 550(i,j) included in the pixel of theinput/output device described in this embodiment is provided so as to beseen in part of a range from which display using the first displayelement 750(i,j) can be seen. For example, dashed arrows shown in FIG.17A denote the directions in which external light is incident on andreflected by the first display element 750(i,j) that performs display bycontrolling the intensity of external light reflection. In addition, asolid arrow shown in FIG. 16A denotes the direction in which the seconddisplay element 550(i,j) emits light to the part of the region fromwhich the display using the first display element 750(i,j) can be seen.

Accordingly, display using the second display element can be seen frompart of the region where the display using the first display element canbe seen. Alternatively, a user can see the display without changing theattitude or the like of the display panel. As a result, a display panelwith high convenience or high reliability can be provided.

The pixel circuit 530(i,j) is electrically connected to the signal lineSig1(j). Note that the conductive film 512A is electrically connected tothe signal line Sig1(j) (see FIG. 17A and FIG. 20). Furthermore, forexample, the transistor in which the second conductive film is used asthe conductive film 512B serving as a source electrode or a drainelectrode can be used as the switch SW1 of the pixel circuit 530(i,j).

The display panel described in this embodiment includes the firstinsulating film 501A (see FIG. 16A).

The first insulating film 501A has a first opening 592A, a secondopening 592B, and an opening 592C (see FIG. 16A or FIG. 17A).

The first opening 592A includes a region overlapping with a firstintermediate film 754A and the first electrode 751(i,j) or a regionoverlapping with the first intermediate film 754A and the secondinsulating film 501C.

The second opening 592B includes a region overlapping with a secondintermediate film 754B and the conductive film 511B. Furthermore, theopening 592C includes a region overlapping with an intermediate film754C and the conductive film 511C.

The first insulating film 501A includes a region that is along an outeredge of the first opening 592A and is between the first intermediatefilm 754A and the second insulating film 501C. The first insulating film501A also includes a region that is along an outer edge of the secondopening 592B and is between the second intermediate film 754B and theconductive film 511B.

The display panel described in this embodiment includes a scan lineG2(i), a wiring CSCOM, a third conductive film ANO, and a signal lineSig2(j) (see FIG. 20).

The second display element 550(i,j) of the display panel described inthis embodiment includes a third electrode 551(i,j), a fourth electrode552, and a layer 5530( ) containing a light-emitting material (see FIG.16A). Note that the third electrode 551(i,j) and the fourth electrode552 are electrically connected to the third conductive film ANO and afourth conductive film VCOM2, respectively (see FIG. 20).

The fourth electrode 552 includes a region overlapping with the thirdelectrode 551(i,j).

The layer 553(j) containing a light-emitting material includes a regionsandwiched between the third electrode 551(i,j) and the fourth electrode552.

The third electrode 551(i,j) is electrically connected to the pixelcircuit 530(i,j) at a connection portion 522.

The first display element 750(i,j) of the display panel described inthis embodiment includes a layer 753 containing a liquid crystalmaterial, the first electrode 751(i,j), and a second electrode 752. Thesecond electrode 752 is positioned such that an electric field thatcontrols the alignment of the liquid crystal material is generatedbetween the second electrode 752 and the first electrode 751(i,j) (seeFIG. 16A and FIG. 17A).

The display panel described in this embodiment includes an alignmentfilm AF1 and an alignment film AF2. The alignment film AF2 is providedsuch that the layer 753 containing a liquid crystal material isinterposed between the alignment film AF1 and the alignment film AF2.

The display panel described in this embodiment includes the firstintermediate film 754A and the second intermediate film 754B.

The first intermediate film 754A includes a region which overlaps withthe second insulating film 501C with the first conductive filminterposed therebetween, and the first intermediate film 754A includes aregion in contact with the first electrode 751(i,j). The secondintermediate film 754B includes a region in contact with the conductivefilm 511B.

The display panel described in this embodiment includes a light-blockingfilm BM, an insulating film 771, a functional film 770P, a functionalfilm 770D, a coloring film CF1, and a coloring film CF2.

The light-blocking film BM has an opening in a region overlapping withthe first display element 750(i,j). The coloring film CF2 is providedbetween the second insulating film 501C and the second display element550(i,j) and includes a region overlapping with the opening 751H (seeFIG. 16A).

The insulating film 771 includes a region sandwiched between thecoloring film CF1 and the layer 753 containing a liquid crystal materialor between the light-blocking film BM and the layer 753 containing aliquid crystal material. The insulating film 771 can reduce unevennessdue to the thickness of the coloring film CF1. Alternatively, impuritiescan be prevented from being diffused from the light-blocking film BM,the coloring film CF1, or the like to the layer 753 containing a liquidcrystal material.

The functional film 770P includes a region overlapping with the firstdisplay element 750(i,j).

The functional film 770D includes a region overlapping with the firstdisplay element 750(i,j). The functional film 770D is provided such thata substrate 770 is interposed between the functional film 770D and thefirst display element 750(i,j). Thus, for example, light reflected bythe first display element 750(i,j) can be diffused.

The display panel described in this embodiment includes a substrate 570,the substrate 770, and a functional layer 520.

The substrate 770 includes a region overlapping with the substrate 570.

The functional layer 520 includes a region sandwiched between thesubstrate 570 and the substrate 770. The functional layer 520 includesthe pixel circuit 530(i,j), the second display element 550(i,j), aninsulating film 521, and an insulating film 528. Furthermore, thefunctional layer 520 includes an insulating film 518 and an insulatingfilm 516 (see FIGS. 16A and 16B).

The insulating film 521 includes a region sandwiched between the pixelcircuit 530(i,j) and the second display element 550(i,j).

The insulating film 528 is provided between the insulating film 521 andthe substrate 570 and has an opening in a region overlapping with thesecond display element 550(i,j).

The insulating film 528 formed along an edge of the third electrode551(i,j) can prevent a short circuit between the third electrode551(i,j) and the fourth electrode.

The insulating film 518 includes a region sandwiched between theinsulating film 521 and the pixel circuit 530(i,j). The insulating film516 includes a region sandwiched between the insulating film 518 and thepixel circuit 530(i,j).

The display panel described in this embodiment includes a bonding layer505, a sealing material 705, and a structure body KB1.

The bonding layer 505 includes a region sandwiched between thefunctional layer 520 and the substrate 570, and has a function ofbonding the functional layer 520 and the substrate 570 together.

The sealing material 705 includes a region sandwiched between thefunctional layer 520 and the substrate 770, and has a function ofbonding the functional layer 520 and the substrate 770 together.

The structure body KB1 has a function of providing a certain spacebetween the functional layer 520 and the substrate 770.

The display panel described in this embodiment includes a terminal 519Band a terminal 519C.

The terminal 519B includes the conductive film 511B and the intermediatefilm 754B, and the intermediate film 754B includes a region in contactwith the conductive film 511B. The terminal 519B is electricallyconnected to the signal line Sig1(j), for example.

The terminal 519B can be electrically connected to a flexible printedcircuit FPC1 using a conductive material ACF1.

The terminal 519C includes the conductive film 511C and the intermediatefilm 754C, and the intermediate film 754C includes a region in contactwith the conductive film 511C. The conductive film 511C is electricallyconnected to the wiring VCOM1, for example.

A conductive material CP is sandwiched between the terminal 519C and thesecond electrode 752, and has a function of electrically connecting theterminal 519C and the second electrode 752. For example, a conductiveparticle can be used as the conductive material CP.

The display panel described in this embodiment includes a driver circuitGD and a driver circuit SD (see FIG. 14A).

The driver circuit GD is electrically connected to the scan line G1(i).The driver circuit GD includes a transistor MD, for example (see FIG.16A). Specifically, a transistor including a semiconductor film that canbe formed in the same process as the transistor included in the pixelcircuit 530(i,j) can be used as the transistor MD.

The driver circuit SD is electrically connected to the signal lineSig1(j). The driver circuit SD is electrically connected to the terminal519B, for example.

«Structure Example of Input Portion»

The input portion includes a region overlapping with the display panel(see FIGS. 14A, 14B-1, 14B-2, and 14C, FIG. 16A, or FIG. 17A).

The input portion includes the substrate 710, a functional layer 720, abonding layer 709, and a terminal 719 (see FIG. 16A and FIG. 17A).

The input portion includes a control line CL(g), a sensor signal lineML(h), and a sensing element 775(g,h) (see FIG. 14B-2).

The functional layer 720 includes a region sandwiched between thesubstrate 770 and the substrate 710. The functional layer 720 includesthe sensing element 775(g,h) and an insulating film 706.

The bonding layer 709 is provided between the functional layer 720 andthe substrate 770 and has a function of bonding the functional layer 720and the substrate 770 together.

The sensing element 775(g,h) is electrically connected to the controlline CL(g) and the sensor signal line ML(h).

The control line CL(g) has a function of supplying a control signal.

The sensing element 775(g,h) receives the control signal and has afunction of supplying the control signal and a sensor signal whichchanges in accordance with a distance between the sensing element775(g,h) and an object approaching a region overlapping with the displaypanel.

The sensor signal line ML(h) has a function of receiving the sensorsignal.

The sensing element 775(g,h) has a light-transmitting property.

The sensing element 775(g,h) includes an electrode C(g) and an electrodeM(h).

The electrode C(g) is electrically connected to the control line CL(g).

The electrode M(h) is electrically connected to the sensor signal lineML(h) and is provided so that an electric field part of which is blockedby an object approaching a region overlapping with the display panel isgenerated between the electrode M(h) and the electrode C(g).

Thus, the object approaching the region overlapping with the displaypanel can be sensed while the image data is displayed on the displaypanel.

The input portion described in this embodiment includes the substrate710 and the bonding layer 709 (see FIG. 16A and FIG. 17A).

The substrate 710 is provided so that the sensing element 775(g,h) issandwiched between the substrate 710 and the substrate 770.

The bonding layer 709 is provided between the substrate 770 and thesensing element 775(g,h) and has a function of bonding the substrate 770and the sensing element 775(g,h) together.

The functional film 770P is provided so that the sensing element775(g,h) is interposed between the functional film 770P and the firstdisplay element 750(i,j). Thus, the intensity of light reflected by thesensing element 775(g,h) can be reduced, for example.

The input portion described in this embodiment includes one group ofsensing elements 775(g,l) to 775(g,q) and another group of sensingelements 775(1,h) to 775(p,h) (see FIG. 19). Note that g is an integergreater than or equal to 1 and less than or equal to p, h is an integergreater than or equal to 1 and less than or equal to q, and each of pand q is an integer greater than or equal to 1.

The one group of the sensing elements 775(g,1) to 775(g,q) includes thesensing element 775(g,h) and is provided in the row direction (indicatedby an arrow Ro2 in the drawing).

The another group of sensing elements 775(1,h) to 775(p,h) includes thesensing element 775(g,h) and is provided in the column direction (thedirection indicated by an arrow Co2 in the drawing) that intersects therow direction.

The one group of sensing elements 775(g,1) to 775(g,q) provided in therow direction includes the electrode C(g) that is electrically connectedto the control line CL(g).

The another group of sensing elements 775(1,h) to 775(p,h) provided inthe column direction includes the electrode M(h) that is electricallyconnected to the sensor signal line ML(h).

The control line CL(g) of the touch panel described in this embodimentincludes a conductive film BR(g,h) (see FIG. 16A). The conductive filmBR(g,h) includes a region overlapping with the sensor signal line ML(h).

The insulating film 706 includes a region sandwiched between the sensorsignal line ML(h) and the conductive film BR(g,h). Thus, a short circuitbetween the sensor signal line ML(h) and the conductive film BR(g,h) canbe prevented.

The touch panel described in this embodiment includes an oscillatorcircuit OSC and a detection circuit DC (see FIG. 19).

The oscillator circuit OSC is electrically connected to the control lineCL(g) and has a function of supplying a control signal. For example, arectangular wave, a sawtooth wave, a triangular wave, or the like can beused as the control signal.

The detection circuit DC is electrically connected to the sensor signalline ML(h) and has a function of supplying a sensor signal on the basisof a change in the potential of the sensor signal line ML(h).

Components of the touch panel are described below. Note that thesecomponents cannot be clearly distinguished and one component may alsoserve as another component or include part of another component.

For example, the first conductive film can be used as the firstelectrode 751(i,j). The first conductive film can be used as areflective film.

In addition, the second conductive film can be used as the conductivefilm 512B serving as a source electrode or a drain electrode of atransistor.

The terminal 719 can be electrically connected to a flexible printedcircuit FPC2 using a conductive material ACF2, for example. In addition,the terminal 719 is electrically connected to the sensing element775(g,h).

«Configuration Example of Pixel Circuit»

A configuration example of a pixel circuit is described with referenceto FIG. 20. The pixel circuit 530(i,j) is electrically connected to thesignal line Sig1(j), the signal line sig2(j), the scan line G1(i), thescan line G2(i), the wiring CSCOM, and the third conductive film ANO.Similarly, the pixel circuit 530(i,j+1) is electrically connected to asignal line Sig1(j+1), a signal line Sig2(i+1), a scan line G1(i), ascan line G2(i), the wiring CSCOM, and the third conductive film ANO.

The pixel circuit 530(i,j) and the pixel circuit 530(i,j+1) each includethe switch SW1 and a capacitor C11.

The pixel circuit 530(i,j) and the pixel circuit 530(i,j+1) each includea switch SW2, a transistor M, and a capacitor C12.

For example, a transistor including a gate electrode electricallyconnected to the scan line G1(i) and a first electrode electricallyconnected to the signal line Sig1(j) can be used as the switch SW1.

The capacitor C11 includes a first electrode electrically connected to asecond electrode of the transistor used as the switch SW1 and a secondelectrode electrically connected to the wiring CSCOM.

For example, a transistor including a gate electrode electricallyconnected to the scan line G2(i) and a first electrode electricallyconnected to the signal line Sig2(i) can be used as the switch SW2.

The transistor M includes a gate electrode electrically connected to asecond electrode of the transistor used as the switch SW2 and a firstelectrode electrically connected to the third conductive film ANO.

Note that a transistor including a conductive film provided such that asemiconductor film is interposed between a gate electrode and theconductive film can be used as the transistor M. For example, as theconductive film, a conductive film electrically connected to a wiringthat can supply the same potential as that of the gate electrode of thetransistor M can be used.

The capacitor C12 includes a first electrode electrically connected tothe second electrode of the transistor used as the switch SW2 and asecond electrode electrically connected to the first electrode of thetransistor M.

Note that in the pixel circuit 530(i,j), the first electrode and thesecond electrode of the first display element 750(i,j) are electricallyconnected to the second electrode of the transistor used as the switchSW1 and the wiring VCOM1, respectively. This enables the first displayelement 750 to be driven. Similarly, in the pixel circuit 530(i,j+1), afirst electrode and a second electrode of the first display element750(i,j+1) are electrically connected to the second electrode of thetransistor used as the switch SW1 and the wiring VCOM1, respectively.This enables the first display element 750 to be driven.

Furthermore, in the pixel circuit 530(i,j), the first electrode and thesecond electrode of the second display element 550(i,j) are electricallyconnected to the second electrode of the transistor M and the fourthconductive film VCOM2, respectively. This enables the second displayelement 550(i,j) to be driven. Similarly, in the pixel circuit530(i,j+1), the first electrode and the second electrode of the seconddisplay element 550(i,j+1) are electrically connected to the secondelectrode of the transistor M and the fourth conductive film VCOM2,respectively. This enables the second display element 550(i,j+1) to bedriven.

«Transistor Structure Example»

A bottom-gate transistor, a top-gate transistor, or the like can be usedas the switch SW1, the transistor M, and the transistor MD.

For example, a transistor including a semiconductor containing anelement belonging to Group 14 can be used. Specifically, a semiconductorcontaining silicon can be used for a semiconductor film. For example, atransistor including single crystal silicon, polysilicon,microcrystalline silicon, amorphous silicon, or the like in asemiconductor film can be used.

For example, a transistor including an oxide semiconductor in asemiconductor film can be used. Specifically, an oxide semiconductorcontaining indium or an oxide semiconductor containing indium, gallium,and zinc can be used for a semiconductor film.

For example, a transistor whose leakage current in an off state issmaller than that of a transistor including amorphous silicon in asemiconductor film can be used as the switch SW1, the transistor M, thetransistor MD, or the like. Specifically, a transistor including anoxide semiconductor in a semiconductor film 508 can be used as theswitch SW1, the transistor M, the transistor MD, or the like.

Thus, a pixel circuit can hold an image signal for a longer time than apixel circuit including a transistor that uses amorphous silicon for asemiconductor film. Specifically, the selection signal can be suppliedat a frequency of lower than 30 Hz, preferably lower than 1 Hz, furtherpreferably less than once per minute while flickering is suppressed.Consequently, eyestrain on a user of the information processing devicecan be reduced, and power consumption for driving can be reduced.

The transistor including an oxide semiconductor in a semiconductor filmwill be described in detail in Embodiment 5.

The transistor that can be used as the switch SW1 includes thesemiconductor film 508 and a conductive film 504 including a regionoverlapping with the semiconductor film 508 (see FIG. 17B). Thetransistor that can be used as the switch SW1 includes the conductivefilm 512A and the conductive film 512B, which are electrically connectedto the semiconductor film 508.

Note that the conductive film 504 and the insulating film 506 serve as agate electrode and a gate insulating film, respectively. The conductivefilm 512A has one of a function as a source electrode and a function asa drain electrode, and the conductive film 512B has the other.

A transistor in which the semiconductor film 508 is provided between theconductive film 504 and a conductive film 524 can be used as thetransistor M (see FIG. 16B).

The above-described input/output device is used for the tablet terminalin FIG. 11A, the mobile phone in FIG. 11B, and the like which aredescribed in Embodiment 3, whereby an electronic device that is highlyvisible, convenient, or reliable can be obtained.

This embodiment can be combined with any of the other embodiments inthis specification as appropriate.

Embodiment 5

In this embodiment, one embodiment of a semiconductor device isdescribed with reference to FIGS. 21A to 21C, FIGS. 22A to 22C, FIGS.23A to 23C, FIGS. 24A to 24C, FIGS. 25A to 25C, FIGS. 26A to 26C, andFIGS. 27A to 27C.

Transistors of one embodiment of the present invention each preferablyinclude an nc-OS or a CAAC-OS, which are described in Embodiment 6.

<Transistor Structure 1>

An example of a transistor of one embodiment of the present invention isdescribed below. FIGS. 21A to 21C are a top view and cross-sectionalviews of the transistor of one embodiment of the present invention. FIG.21A is a top view. FIG. 21B is a cross-sectional view taken alongdashed-dotted line X1-X2 in FIG. 21A. FIG. 21C is a cross-sectional viewtaken along dashed-dotted line Y1-Y2 in FIG. 21A. Note that thedirection of the dashed-dotted line X1-X2 may be called the channellength direction, and the direction of the dashed-dotted line Y1-Y2 maybe called the channel width direction. Note that for simplification ofthe drawing, some components in the top view in FIG. 21A are notillustrated.

A transistor 1200A includes a conductor 1205 (a conductor 1205 a and aconductor 1205 b) functioning as a back gate electrode, a conductor 1260functioning as a gate electrode, an insulator 1220 functioning as a gateinsulating layer, an insulator 1222, an insulator 1224, an insulator1250, an oxide 1230 having a region where a channel is formed (an oxide1230 a, an oxide 1230 b, and an oxide 1230 c), a conductor 1240 afunctioning as one of a source and a drain, a conductor 1240 bfunctioning as the other of the source and the drain, and an insulator1280 containing excess oxygen, and an insulator 1282 having a barrierproperty.

The oxide 1230 includes the oxide 1230 a, the oxide 1230 b over theoxide 1230 a, and the oxide 1230 c over the oxide 1230 b. When thetransistor 1200A is turned on, current flows (a channel is formed)mainly in the oxide 1230 b. n contrast, although current might flowthrough a region in the vicinity of the interface (a mixed region insome cases) between the oxide 1230 b and the oxides 1230 a and 1230 c,the rest of the oxides 1230 a and 1230 c might function as insulators atthe other region.

As illustrated in FIGS. 21A to 21C, the oxide 1230 c is preferablyprovided to cover side surfaces of the oxides 1230 a and 1230 b. Theoxide 1230 c, which is provided between the insulator 1280 and the oxide1230 b including the region where the channel is formed, can preventimpurities such as hydrogen, water, and halogen from diffusing from theinsulator 1280 into the oxide 1230 b.

A metal film containing an element selected from molybdenum, titanium,tantalum, tungsten, aluminum, copper, chromium, neodymium, and scandium;a metal nitride film containing any of the elements as its component(e.g., a tantalum nitride film, a titanium nitride film, a molybdenumnitride film, or a tungsten nitride film); or the like is used as theconductor 1205 functioning as a back gate electrode. In particular, ametal nitride film such as a tantalum nitride film is preferable becauseit has a barrier property against hydrogen or oxygen and is difficult tooxidize (has high oxidation resistance). Alternatively, a conductivematerial such as indium tin oxide, indium oxide containing tungstenoxide, indium zinc oxide containing tungsten oxide, indium oxidecontaining titanium oxide, indium tin oxide containing titanium oxide,indium zinc oxide, or indium tin oxide to which silicon oxide is addedcan also be used.

For example, a conductor having a barrier property against hydrogen,e.g., tantalum nitride, may be used as the conductor 1205 a, andtungsten, which has high conductivity, may be stacked thereover as theconductor 1205 b. The use of the combination of the materials canprevent diffusion of hydrogen into the oxide 1230 while conductivity ofa wiring is ensured. A two-layer structure of the conductor 1205 a andthe conductor 1205 b is shown in FIGS. 21A to 21C, but the structure ofthe conductor 1205 is not limited thereto, and a single-layer structureor a layered structure of three or more layers may be used. For example,a structure where a conductor having a barrier property and a conductorwhich has high conductivity are provided with a conductor which ishighly adhesive to the conductor having a barrier property and theconductor which has high conductivity located therebetween may beemployed.

Each of the insulators 1220 and 1224 is preferably an insulatorcontaining oxygen, such as a silicon oxide film or a silicon oxynitridefilm. In particular, the insulator 1224 is preferably an insulatorcontaining excess oxygen (containing oxygen in excess of that in thestoichiometric composition). In the case where such an insulatorcontaining excess oxygen is provided in contact with the oxide 1230 inthe transistor 1200A, oxygen vacancies in the oxide 1230 can becompensated. Note that the insulators 1222 and 1224 are not necessarilyformed of the same material.

The insulator 1222 is preferably formed to have a single-layer structureor a stacked-layer structure using, for example, an insulator containinga material such as silicon oxide, silicon oxynitride, silicon nitrideoxide, aluminum oxide, hafnium oxide, tantalum oxide, zirconium oxide,lead zirconate titanate (PZT), strontium titanate (SrTiO₃), or(Ba,Sr)TiO₃ (BST). In particular, an insulating film having a barrierproperty against oxygen or hydrogen, e.g., an aluminum oxide film or ahafnium oxide film, is preferably used. The insulator 1222 formed ofsuch a material functions as a layer that prevents release of oxygenfrom the metal oxide 1230 or entry of impurities such as hydrogen fromthe outside.

Alternatively, aluminum oxide, bismuth oxide, germanium oxide, niobiumoxide, silicon oxide, titanium oxide, tungsten oxide, yttrium oxide, orzirconium oxide may be added to the insulator, for example. Theinsulator may be subjected to nitriding treatment. A layer of siliconoxide, silicon oxynitride, or silicon nitride may be stacked over theinsulator.

Note that each of the insulators 1220, 1222, and 1224 may have astacked-layer structure including two or more layers. In this case, thestacked layers are not necessarily formed of the same material but maybe formed of different materials.

In the case where the insulator 1222 including a high-k material isprovided between the insulator 1220 and the insulator 1224, electronscan be trapped in the insulator 1222 under specific conditions, and thethreshold voltage can be increased. As a result, the insulator 1222 isnegatively charged in some cases.

For example, in the case where the insulator 1220 and the insulator 1224are formed using silicon oxide and the insulator 1222 is formed using amaterial having a lot of electron trap states such as hafnium oxide,aluminum oxide, or tantalum oxide, the state where the potential of theconductor 1205 is higher than the potential of the source electrode andthe drain electrode is kept at a temperature higher than the operatingtemperature or the storage temperature of the semiconductor device(e.g., at a temperature higher than or equal to 125° C. and lower thanor equal to 450° C., typically higher than or equal to 150° C. and lowerthan or equal to 300° C.) for 10 milliseconds or longer, typically oneminute or longer. Thus, electrons are moved from the oxide 1230 in thetransistor 1200A to the conductor 1205. At this time, some of the movingelectrons are trapped by the electron trap states of the insulator 1222.

In the transistor in which a necessary amount of electrons is trapped bythe electron trap states of the insulator 1222, the threshold voltage isshifted in the positive direction. By controlling the voltage of theconductor 1205, the amount of electrons to be trapped can be controlled,and thus the threshold voltage can be controlled. The transistor 1200Ahaving the structure is a normally-off transistor which is in anon-conducting state (also referred to as an off state) even when thegate voltage is 0 V.

The treatment for trapping the electrons may be performed in themanufacturing process of the transistor. For example, the treatment ispreferably performed at any step before factory shipment, such as afterthe formation of a conductor connected to a source conductor or a drainconductor of the transistor, after the preceding process (waferprocessing), after a wafer-dicing step, or after packaging.

The threshold voltages can be controlled by appropriate adjustment ofthe thicknesses of the insulator 1220, the insulator 1222, and theinsulator 1224. For example, when the total thickness of the insulator1220, the insulator 1222, and the insulator 1224 is reduced, a voltageis efficiently applied from the conductor 1205, resulting in low powerconsumption of the transistor. The total thickness of the insulators1220, 1222, and 1224 is less than or equal to 65 nm, preferably lessthan or equal to 20 nm.

Thus, a transistor having a low leakage current in an off state can beprovided. A transistor with stable electrical characteristics can beprovided. A transistor having high on-state current can be provided. Atransistor having a small subthreshold swing value can be provided. Ahighly reliable transistor can be provided.

The oxide 1230 a, the oxide 1230 b, and the oxide 1230 c are formedusing a metal oxide such as an In-M-Zn oxide (M is Al, Ga, Y, or Sn).Alternatively, an In—Ga oxide or an In—Zn oxide may be used for theoxide 1230.

The oxide 1230 according to the present invention is described below.

An oxide used for the oxide 1230 preferably contains at least indium orzinc. In particular, indium and zinc are preferably contained. Inaddition, aluminum, gallium, yttrium, tin, or the like is preferablycontained. Furthermore, one or more elements selected from boron,silicon, titanium, iron, nickel, germanium, zirconium, molybdenum,lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, orthe like may be contained.

Here, the case where an oxide contains indium, an element M, and zinc isconsidered. The element M is aluminum, gallium, yttrium, tin, or thelike. Other elements which can be used as the element M are boron,silicon, titanium, iron, nickel, germanium, yttrium, zirconium,molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten,magnesium, and the like. Note that two or more of the above elements maybe used in combination as the element M.

Next, preferred ranges of atomic ratios of indium to the element M andzinc in the oxide of one embodiment of the present invention aredescribed with reference to FIGS. 28A to 28C. Note that the proportionof oxygen atoms is not illustrated in FIGS. 28A to 28C. The terms of theatomic ratio of indium to the element M and zinc in the oxide aredenoted by [In], [M], and [Zn], respectively.

In FIGS. 28A to 28C, dashed lines indicate a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):1 where α is a real number greater than orequal to −1 and less than or equal to 1, a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):2, a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):3, a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):4, and a line where the atomic ratio[In]:[M]:[Zn] is (1+α):(1−α):5.

Dashed-dotted lines indicate a line where the atomic ratio [In]:[M]:[Zn]is 1:1:β, where β is a real number greater than or equal to 0, a linewhere the atomic ratio [In]:[M]:[Zn] is 1:2:β, a line where the atomicratio [In]:[M]: [Zn] is 1:3:β, a line where the atomic ratio[In]:[M]:[Zn] is 1:4:β, a line where the atomic ratio [In]:[M]:[Zn] is2:1:β, and a line where the atomic ratio [In]:[M]1:[Zn] is 5:1:β.

The metal oxide in FIGS. 28A to 28C with an atomic ratio of[In]:[M]:[Zn]=0:2:1 or an atomic ratio that is in the neighborhoodthereof is likely to have a spinel crystal structure.

FIGS. 28A and 28B illustrate examples of the preferred ranges of theatomic ratios of indium to the element M and zinc contained in an oxideof one embodiment of the present invention.

As an example, FIG. 29 shows a crystal structure of InMZnO₄ whose atomicratio [In]: [M]: [Zn] is 1:1:1. The crystal structure illustrated inFIG. 29 is InMZnO₄ observed from a direction parallel to a b-axis. Notethat a metal element in a layer that contains M, Zn, and oxygen(hereinafter, this layer is referred to as an “(M, Zn) layer”) in FIG.29 represents the element M or zinc. In that case, the proportion of theelement M is the same as the proportion of zinc. The element M and zinccan be replaced with each other, and their arrangement is random.

Note that InMZnO₄ has a layered crystal structure (also referred to as alayered structure) and includes one layer that contains indium andoxygen (hereinafter referred to as an In layer) for every two (M,Zn)layers that contain the element M, zinc, and oxygen, as illustrated inFIG. 29.

Indium and the element M can be replaced with each other. Therefore,when the element M in the (M,Zn) layer is replaced with indium, thelayer can also be referred to as an (In,M,Zn) layer. In that case, alayered structure that contains one In layer for every two (In,M,Zn)layers is obtained.

An oxide whose atomic ratio [In]:[M]:[Zn] is 1:1:2 has a layeredstructure that includes three (M,Zn) layers with respect to one Inlayer. In other words, if [Zn] is larger than [In] and [M], theproportion of (M,Zn) layers to In layers becomes higher when the oxideis crystallized.

Note that in the case where the number of (M,Zn) layers with respect toone In layer is not an integer in the oxide, the oxide might have pluralkinds of layered structures where the number of (M,Zn) layers withrespect to one In layer is an integer. For example, in the case of[In]:[M]:[Zn]=1:1:1.5, the oxide might have the following layeredstructures: a layered structure of two (M,Zn) layers with respect to oneIn layer and a layered structure of three (M,Zn) layers with respect toone In layer.

For example, in the case where the oxide is deposited with a sputteringapparatus, a film having an atomic ratio deviated from the atomic ratioof a target is formed. In particular, [Zn] in the film might be lowerthan [Zn] in the target depending on the substrate temperature indeposition.

A plurality of phases (e.g., two phases or three phases) exists in theoxide in some cases. For example, with an atomic ratio [In]:[M]:[Zn]that is close to 0:2:1, two phases of a spinel crystal structure and alayered crystal structure are likely to exist. In addition, with anatomic ratio [In]:[M]:[Zn] that is close to 1:0:0, two phases of abixbyite crystal structure and a layered crystal structure are likely toexist. In the case where a plurality of phases exists in the oxide, agrain boundary might be formed between different crystal structures.

In addition, the oxide containing indium in a higher proportion can havea higher carrier mobility (electron mobility). This is because in anoxide containing indium, the element M, and zinc, the s orbital of heavymetal mainly contributes to carrier transfer, and when the indiumcontent in the oxide is increased, overlaps of the s orbitals of indiumatoms are increased; therefore, an oxide having a high content of indiumhas a higher carrier mobility than an oxide having a low content ofindium.

In contrast, when the indium content and the zinc content in an oxidebecome lower, carrier mobility becomes lower. Thus, with atomic ratiosof [In]:[M]:[Zn]=0:1:0 and the neighborhood thereof (e.g., a region C inFIG. 28C), insulation performance becomes better.

Accordingly, an oxide of one embodiment of the present inventionpreferably has an atomic ratio represented by a region A in FIG. 28A.With the atomic ratio, a layered structure with a high carrier mobilityand a few grain boundaries is easily obtained.

A region B in FIG. 28B represents an atomic ratio of [In]:[M]:[Zn]=4:2:3or 4:2:4.1 and the neighborhood thereof. The neighborhood includes anatomic ratio of [In]:[M]:[Zn]=5:3:4. An oxide with an atomic ratiorepresented by the region B is an excellent oxide that has particularlyhigh crystallinity and high carrier mobility.

Note that the condition where an oxide forms a layered structure is notuniquely determined by an atomic ratio. There is a difference in thedegree of difficulty in forming a layered structure among atomic ratios.Even with the same atomic ratio, whether a layered structure is formedor not depends on a formation condition. Therefore, the illustratedregions each represent an atomic ratio with which an oxide has a layeredstructure, and boundaries of the regions A to C are not clear.

Next, the case where the oxide is used for a transistor is described.

Note that when the oxide is used for a transistor, carrier scattering orthe like at a grain boundary can be reduced; thus, the transistor canhave high field-effect mobility. In addition, the transistor can havehigh reliability.

An oxide with a low carrier density is preferably used for thetransistor. For example, an oxide whose carrier density is lower than8×10¹¹ cm³, preferably lower than 1×10¹¹ cm³, further preferably lowerthan 1×10¹⁰ cm³, and higher than or equal to 1×10⁻⁹ cm³ is used.

Note that a highly purified intrinsic or substantially highly purifiedintrinsic metal oxide has few carrier generation sources, and thus has alow carrier density. A highly purified intrinsic or substantially highlypurified intrinsic metal oxide has a low density of defect states andaccordingly has low density of trap states in some cases.

Charge trapped by the trap states in the oxide takes a long time to bereleased and may behave like fixed charge. Thus, a transistor whosechannel region is formed in an oxide having a high density of trapstates has unstable electrical characteristics in some cases.

In order to obtain stable electrical characteristics of the transistor,it is effective to reduce the concentration of impurities in the oxide.In addition, in order to reduce the concentration of impurities in theoxide, the concentration of impurities in a film that is adjacent to theoxide is preferably reduced. As examples of the impurities, hydrogen,nitrogen, alkali metal, alkaline earth metal, iron, nickel, silicon, andthe like are given.

Here, the influence of impurities in the oxide is described.

When silicon or carbon that is a Group 14 element is contained in theoxide, defect states are formed. Thus, the concentration of silicon orcarbon (measured by secondary ion mass spectrometry (SIMS)) is set to belower than or equal to 2×10¹⁸ atoms/cm³, preferably lower than or equalto 2×10¹⁷ atoms/cm³ in the oxide or around an interface with the oxide.

When the metal oxide contains alkali metal or alkaline earth metal,defect states are formed and carriers are generated, in some cases.Thus, a transistor including an oxide that contains alkali metal oralkaline earth metal is likely to be normally-on. Therefore, it ispreferable to reduce the concentration of alkali metal or alkaline earthmetal in the oxide. Specifically, the concentration of alkali metal oralkaline earth metal in the metal oxide, which is measured by SIMS, islower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equalto 2×10¹⁶ atoms/cm³.

When containing nitrogen, the oxide easily becomes n-type by generationof electrons serving as carriers and an increase of carrier density.Thus, a transistor whose semiconductor includes an oxide containingnitrogen is likely to be normally-on. For this reason, nitrogen in theoxide is preferably reduced as much as possible; for example, theconcentration of nitrogen in the oxide measured by SIMS is set to belower than 5×10¹⁹ atoms/cm³, preferably lower than or equal to 5×10¹⁸atoms/cm³, further preferably lower than or equal to 1×10¹⁸ atoms/cm³,and still further preferably lower than or equal to 5×10¹⁷ atoms/cm³.

Hydrogen contained in an oxide reacts with oxygen bonded to a metal atomto be water, and thus causes an oxygen vacancy in some cases. Entry ofhydrogen into the oxygen vacancy generates an electron serving as acarrier in some cases. Furthermore, in some cases, bonding of part ofhydrogen to oxygen bonded to a metal atom causes generation of anelectron serving as a carrier. Thus, a transistor including an oxidethat contains hydrogen is likely to be normally-on. Accordingly,hydrogen in the oxide is preferably reduced as much as possible.Specifically, the concentration of hydrogen in the oxide measured bySIMS is set to be lower than 1×10²⁰ atoms/cm³, preferably lower than1×10¹⁹ atoms/cm³, further preferably lower than 5×10¹⁸ atoms/cm³, andstill further preferably lower than 1×10¹⁸ atoms/cm³.

When an oxide with sufficiently reduced impurity concentration is usedfor a channel region in a transistor, the transistor can have stableelectrical characteristics.

Next, the case where the oxide has a two-layer structure or athree-layer structure is described. A band diagram of a stacked-layerstructure of an oxide S1, an oxide S2, and an oxide S3 and insulatorsthat are in contact with the stacked-layer structure, a band diagram ofa stacked-layer structure of the oxides S2 and S3 and insulators thatare in contact with the stacked-layer structure, and a band diagram of astacked-layer structure of the oxides S1 and S2 and insulators that arein contact with the stacked-layer structure are described with referenceto FIGS. 30A to 30C.

FIG. 30A is an example of a band diagram of a stacked structureincluding an insulator I1, the oxide S1, the oxide S2, the oxide S3, andan insulator 12 in a thickness direction. FIG. 30B is an example of aband diagram of a stacked structure including the insulator I1, theoxide S2, the oxide S3, and the insulator 12 in a thickness direction.FIG. 30C is an example of a band diagram of a stacked structureincluding the insulator I1, the oxide semiconductor S1, the oxidesemiconductor S2, and the insulator 12 in a thickness direction. Notethat for easy understanding, the band diagrams show the energy level ofthe conduction band minimum (Ec) of each of the insulator I1, the oxideS1, the oxide S2, the oxide S3, and the insulator 12.

The energy level of the conduction band minimum of each of the oxides S1and S3 is closer to the vacuum level than that of the oxide S2 is.Typically, the conduction band minimum of the oxide S2 is lower than theconduction band minimum of each of the oxide S1 and the oxide S3.Specifically, a difference in the energy level between the conductionband minimum of the oxide S2 and the conduction band minimum of each ofthe oxides S1 and S3 is preferably greater than or equal to 0.15 eV andless than or equal to 2 eV, further preferably greater than or equal to0.5 eV and less than or equal to 1 eV. That is, it is preferable thatthe electron affinity of the oxide S2 be higher than the electronaffinity of each of the oxides S1 and S3, and the difference between theelectron affinity of each of the oxides S1 and S3 and the electronaffinity of the oxide S2 be greater than or equal to 0.15 eV and lessthan or equal to 2 eV, further preferably greater than or equal to 0.5eV and less than or equal to 1 eV.

As shown in FIGS. 30A to 30C, the energy level of the conduction bandminimum of each of the oxides S1 to S3 is gradually varied. In otherwords, the energy level of the conduction band minimum is continuouslychanged or continuously connected. In order to obtain such a banddiagram, the density of defect states in a mixed layer formed at aninterface between the oxide semiconductors S1 and S2 or an interfacebetween the oxide semiconductors S2 and S3 is preferably made low.

Specifically, when the oxides S1 and S2 or the oxides S2 and S3 containthe same element (as a main component) in addition to oxygen, a mixedlayer with a low density of defect states can be formed. For example, inthe case where the oxide S2 is an In—Ga—Zn oxide, it is preferable touse an In—Ga—Zn oxide, a Ga—Zn oxide, gallium oxide, or the like as eachof the oxides S1 and S3.

At this time, the oxide S2 serves as a main carrier path. Since thedensity of defect states at the interface between the oxide S1 and theoxide S2 and the interface between the oxide S2 and the oxide S3 can bemade low, the influence of interface scattering on carrier conduction issmall, and a high on-state current can be obtained.

When an electron is trapped in a trap state, the trapped electronbehaves like fixed charge; thus, the threshold voltage of the transistoris shifted in a positive direction. The oxides S1 and S3 can make thetrap state apart from the oxide S2. This structure can prevent thepositive shift of the threshold voltage of the transistor.

A material whose conductivity is sufficiently lower than that of theoxide S2 is used for the oxides S1 and S3. In that case, the oxide S2,the interface between the oxide S1 and the oxide S2, and the interfacebetween the oxide S2 and the oxide S3 mainly function as a channelregion. For example, an oxide with high insulation performance and theatomic ratio represented by the region C in FIG. 28C may be used as eachof the oxides S1 and S3. Note that the region C in FIG. 28C representsthe atomic ratio of [In]:[M]:[Zn]=0:1:0 or the neighborhood thereof.

In the case where an oxide with the atomic ratio represented by theregion A is used as the oxide S2, it is particularly preferable to usean oxide with an atomic ratio where [M]/[In] is greater than or equal to1, preferably greater than or equal to 2, as each of the oxide S1 andthe oxide S3. In addition, it is suitable to use an oxide withsufficiently high insulation performance and an atomic ratio where[M]/([Zn]+[In]) is greater than or equal to 1 as the oxide S3.

The insulator 1250 can have a single-layer structure or a stacked-layerstructure using, for example, an insulator containing a so-called high-kmaterial such as silicon oxide, silicon oxynitride, silicon nitrideoxide, aluminum oxide, hafnium oxide, tantalum oxide, zirconium oxide,lead zirconate titanate (PZT), strontium titanate (SrTiO₃), or(Ba,Sr)TiO₃ (BST). Aluminum oxide, bismuth oxide, germanium oxide,niobium oxide, silicon oxide, titanium oxide, tungsten oxide, yttriumoxide, or zirconium oxide may be added to the insulator, for example.The insulator may be subjected to nitriding treatment. A layer ofsilicon oxide, silicon oxynitride, or silicon nitride may be stackedover the insulator.

As the insulator 1250, like the insulator 1224, an oxide insulator thatcontains oxygen in excess of that in the stoichiometric composition ispreferably used. When such an insulator containing excess oxygen isprovided in contact with the oxide 1230, oxygen vacancies in the oxide1230 can be reduced.

As the insulator 1250, an insulating film formed of aluminum oxide,aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide,yttrium oxynitride, hafnium oxide, hafnium oxynitride, silicon nitride,or the like, which has barrier properties against oxygen and hydrogen,can be used. The insulator 1250 formed of such a material functions as alayer that prevents release of oxygen from the oxide 1230 or entry ofimpurities such as hydrogen from the outside.

Note that the insulator 1250 may have a stacked-layer structure similarto that of the insulator 1220, the insulator 1222, and the insulator1224. When the insulator 1250 includes an insulator in which a necessaryamount of electrons is trapped by electron trap states, the thresholdvoltage of the transistor 1200A can be shifted in the positivedirection. The transistor 1200A having the structure is a normally-offtransistor which is in a non-conducting state (also referred to as anoff state) even when the gate voltage is 0 V.

In addition to the insulator 1250, a barrier film may be providedbetween the oxide 1230 and the conductor 1260 in the semiconductordevice illustrated in FIGS. 21A to 21C. The oxide 1230 c may have abarrier property.

For example, an insulating film containing excess oxygen is provided incontact with the oxide 1230 and covered by a barrier film, whereby thecomposition of the oxide can be almost the same as the stoichiometriccomposition or can be in a supersaturated state containing more oxygenthan that in the stoichiometric composition. It is also possible toprevent entry of impurities such as hydrogen into the oxide 1230.

One of a pair of the conductor 1240 a and the conductor 1240 b functionsas a source electrode, and the other functions as a drain electrode.

Any of metals such as aluminum, titanium, chromium, nickel, copper,yttrium, zirconium, molybdenum, silver, tantalum, and tungsten, or analloy containing any of the metals as its main component can be used foreach of the conductors 1240 a and 1240 b. In particular, a metal nitridefilm such as a tantalum nitride film is preferable because it has abarrier property against hydrogen or oxygen and has high oxidationresistance.

Although a single-layer structure is illustrated in the drawings, astacked-layer structure of two or more layers may be used. For example,a tantalum nitride film and a tungsten film may be stacked.Alternatively, a titanium film and an aluminum film may be stacked.Other examples include a two-layer structure where an aluminum film isstacked over a tungsten film, a two-layer structure where a copper filmis stacked over a copper-magnesium-aluminum alloy film, a two-layerstructure where a copper film is stacked over a titanium film, and atwo-layer structure where a copper film is stacked over a tungsten film.

Other examples include a three-layer structure where a titanium film ora titanium nitride film is formed, an aluminum film or a copper film isstacked over the titanium film or the titanium nitride film, and atitanium film or a titanium nitride film is formed over the aluminumfilm or the copper film; and a three-layer structure where a molybdenumfilm or a molybdenum nitride film is formed, an aluminum film or acopper film is stacked over the molybdenum film or the molybdenumnitride film, and a molybdenum film or a molybdenum nitride film isformed over the aluminum film or the copper film. Note that atransparent conductive material containing indium oxide, tin oxide, orzinc oxide may be used.

The conductor 1260 functioning as a gate electrode can be formed using,for example, a metal selected from aluminum, chromium, copper, tantalum,titanium, molybdenum, and tungsten, an alloy containing any of thesemetals as its component, an alloy containing any of these metals incombination, or the like. In particular, a metal nitride film such as atantalum nitride film is preferable because it has a barrier propertyagainst hydrogen or oxygen and has high oxidation resistance.Furthermore, one or both of manganese and zirconium may be used.Alternatively, a semiconductor typified by polycrystalline silicon dopedwith an impurity element such as phosphorus, or a silicide such asnickel silicide may be used. Although a single-layer structure isillustrated in the drawings, a stacked-layer structure of two or morelayers may be used.

A two-layer structure where a titanium film is stacked over an aluminumfilm may be employed, for example. Other examples include a two-layerstructure where a titanium film is stacked over a titanium nitride film,a two-layer structure where a tungsten film is stacked over a titaniumnitride film, and a two-layer structure where a tungsten film is stackedover a tantalum nitride film or a tungsten nitride film.

Other examples include a three-layer structure where a titanium film isformed, an aluminum film is stacked over the titanium film, and atitanium film is formed over the aluminum film. An alloy film or anitride film that contains aluminum and one or more metals selected fromtitanium, tantalum, tungsten, molybdenum, chromium, neodymium, andscandium may be used.

The conductor 1260 can also be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded. The conductor 1260 can have a stacked structure using the abovelight-transmitting conductive material and the above metal.

Next, the insulator 1280 and the insulator 1282 are provided over thetransistor 1200A.

The insulator 1280 preferably contains an oxide containing oxygen morethan that in the stoichiometric composition. That is, in the insulator1280, a region containing oxygen in excess of that in the stoichiometriccomposition (hereinafter also referred to as oxygen-excess region) ispreferably formed. In particular, in the case of using an oxidesemiconductor in the transistor 1200A, when an insulator including anexcess-oxygen region is provided in an interlayer film or the like inthe vicinity of the transistor 1200A, oxygen vacancies in the oxide 1230of the transistor 1200A are reduced, whereby the reliability can beimproved.

As the insulator including the excess-oxygen region, specifically, anoxide material that releases part of oxygen by heating is preferablyused. Oxide that releases part of oxygen by heating is an oxide film ofwhich the amount of released oxygen converted into oxygen atoms isgreater than or equal to 1.0×10¹⁸ atoms/cm³, preferably greater than orequal to 3.0×10²⁰ atoms/cm³ in TDS analysis. Note that the temperatureof the film surface in the TDS analysis is preferably higher than orequal to 100° C. and lower than or equal to 700° C., or higher than orequal to 100° C. and lower than or equal to 500° C.

For example, as such a material, a material containing silicon oxide orsilicon oxynitride is preferably used. Alternatively, a metal oxide canbe used. Note that in this specification, “silicon oxynitride” refers toa material that contains oxygen at a higher proportion than nitrogen,and “silicon nitride oxide” refers to a material that contains nitrogenat a higher proportion than oxygen.

The insulator 1280 covering the transistor 1200A may function as aplanarization film that covers a roughness thereunder.

The insulator 1282 is preferably formed using an insulating film havinga barrier property against oxygen or hydrogen, e.g., an aluminum oxidefilm or hafnium oxide film. The insulator 1282 formed of such a materialfunctions as a layer that prevents release of oxygen from the metaloxide 1230 or entry of impurities such as hydrogen from the outside.

The above structure makes it possible to provide a transistor includingan oxide semiconductor with high on-state current. A transistorincluding an oxide semiconductor with low off-state current can beprovided. Furthermore, when the transistor with the above structure isused in a semiconductor device, variation in the electricalcharacteristics of the semiconductor device can be reduced, and thereliability thereof can be improved. Furthermore, a semiconductor devicewith low power consumption can be provided.

<Transistor Structure 2>

FIGS. 22A to 22C illustrate an example of a transistor structuredifferent from that in FIGS. 21A to 21C. FIG. 22A illustrates a topsurface of a transistor 1200B. Some films are omitted in FIG. 22A forsimplification of the drawing. FIG. 22B is a cross-sectional view alongthe dashed-dotted line X1-X2 in FIG. 22A. FIG. 22C is a cross-sectionalview along the dashed-dotted line Y1-Y2 in FIG. 22A. Note that thedirection of the dashed-dotted line X1-X2 may be called the channellength direction, and the direction of the dashed-dotted line Y1-Y2 maybe called the channel width direction.

Note that in the transistor 1200B illustrated in FIGS. 22A to 22C,components having the same function as the components in the transistor1200A in FIGS. 21A to 21C are denoted by the same reference numerals.

In the structure in FIGS. 22A to 22C, the conductor 1260 has two stackedlayers of a conductor 1260 a and a conductor 1260 b. In the two-layerstructure, layers formed using the same material may be stacked. Forexample, the conductor 1260 a is formed by a thermal CVD method, anMOCVD method, or an ALD method. In particular, the conductor 1260 a ispreferably formed by an ALD method. The application of an ALD method orthe like can reduce damage to the insulator 1250 which is caused duringdeposition. Furthermore, it is preferable to form the conductor 1260 aby an ALD method or the like, because coverage can be improved. Thus,the transistor with high reliability can be provided.

Next, the conductor 1260 b is formed by a sputtering method. At thattime, since the conductor 1260 a is provided over the insulator 1250,damage caused during deposition of the conductor 1260 b can be preventedfrom affecting the insulator 1250. Since the deposition rate in asputtering method is higher than that in an ALD method, the productivitycan be improved with a high yield.

In the structure illustrated in FIGS. 22A to 22C, an insulator 1270 isprovided to cover the conductor 1260. In the case where the insulator1280 is formed using an oxide material from which oxygen is released,the insulator 1270 is formed using a substance having a barrier propertyagainst oxygen to prevent the conductor 1260 b from being oxidized bythe released oxygen.

For example, the insulator 1270 can be formed using metal oxide such asaluminum oxide. The insulator 1270 is formed to a thickness with whichthe oxidation of the conductor 1260 is prevented. For example, thethickness of the insulator 1270 is set greater than or equal to 1 nm andless than or equal to 10 nm, preferably greater than or equal to 3 nmand less than or equal to 7 nm.

This structure can expand the range of choices for materials of theconductor 1260. For example, a material which has high conductivitywhile having low oxidation resistance, such as aluminum, can be used.Furthermore, a conductor that can be easily formed or processed can beused, for example.

Thus, the oxidation of the conductor 1260 can be prevented, and oxygenreleased from the insulator 1280 can be supplied to the oxide 1230efficiently. Furthermore, a conductor that has high conductivity is usedfor the conductor 1260, whereby the transistor with low powerconsumption can be provided.

<Transistor Structure 3>

FIGS. 23A to 23C illustrate an example of a transistor structuredifferent from those in FIGS. 21A to 21C and FIGS. 22A to 22C. FIG. 23Aillustrates a top surface of a transistor 1200C. Some films are omittedin FIG. 23A for simplification of the drawing. FIG. 23B is across-sectional view along the dashed-dotted line X1-X2 in FIG. 23A.FIG. 23C is a cross-sectional view along the dashed-dotted line Y1-Y2 inFIG. 23A. Note that the direction of the dashed-dotted line X1-X2 may becalled the channel length direction, and the direction of thedashed-dotted line Y1-Y2 may be called the channel width direction.

Note that in the transistor 1200C in FIGS. 23A to 23C, components havingthe same function as the components in the transistor 1200A in FIGS. 21Ato 21C are denoted by the same reference numerals.

In the structure illustrated in FIGS. 23A to 23C, the conductor 1260functioning as the gate electrode includes a conductor 1260 a, aconductor 1260 b, and a conductor 1260 c. The oxide 1230 c over theinsulator 1224 may be divided as long as it covers the side surface ofthe oxide 1230 b.

The conductor 1260 a is formed by a thermal CVD method, an MOCVD method,and an ALD method. In particular, the conductor 1260 a is preferablyformed by an ALD method. The use of an ALD method or the like can reduceplasma damage to the insulator 1250. Furthermore, it is preferable toform the conductor 1260 a by an ALD method or the like, because coveragecan be improved. Thus, the transistor with high reliability can beprovided.

The conductor 1260 b is formed using a material having highconductivity, such as tantalum, tungsten, copper, or aluminum. Theconductor 1260 c, which is formed over the conductor 1260 b, ispreferably formed using a material with high oxidation resistance, suchas tungsten nitride.

For example, in the case where an oxide material from which oxygen isreleased is used as the insulator 1280, when a conductor with highoxidation resistance is used as the conductor 1260 c, a large area ofwhich is in contact with the insulator 1280 including the oxygen-excessregion, oxygen released from the oxygen-excess region can be preventedfrom being absorbed by the conductor 1260. Thus, the oxidation of theconductor 1260 can be prevented, and oxygen released from the insulator1280 can be supplied to the metal oxide 1230 efficiently. Furthermore, aconductor that has high conductivity is used for the conductor 1260 b,whereby the transistor with low power consumption can be provided.

As illustrated in FIG. 23C, the oxide 1230 b is covered with theconductor 1260 in the channel width direction of the transistor. By theexistence of a projection of the insulator 1224, the side surface of theoxide semiconductor 1230 b can be covered with the conductor 1260. Forexample, at the side surfaces of the oxide 1230 b, the bottom surface ofthe conductor 1260 is preferably located closer to the substrate thanthe bottom surface of the oxide 1230 b by adjusting the shape of theprojection of the insulator 1224. That is, the transistor 1200C has astructure in which the oxide 1230 b can be electrically surrounded by anelectric field of the conductor 1260. Such a structure in which theoxide 1230 b is electrically surrounded by an electric field of theconductors is referred to as a surrounded channel (s-channel) structure.In the s-channel structure of the transistor 1200C, a channel can beformed in the whole (bulk) of the oxide 1230 b. In the s-channelstructure, the drain current of the transistor is increased, so that alarger amount of on-state current (current which flows between thesource and the drain when the transistor is on) can be obtained.Furthermore, the entire channel formation region of the oxide 1230 b canbe depleted by the electric field of the conductor 1260. Accordingly,the off-state current of the s-channel transistor can be furtherreduced. When the channel width is shortened, the effects of thes-channel structure, such as an increase in on-state current and areduction in off-state current, can be enhanced.

<Transistor Structure 4>

FIGS. 24A to 24C illustrate an example of a transistor structuredifferent from those in FIGS. 21A to 21C, FIGS. 22A to 22C, and FIGS.23A to 23C. FIG. 24A illustrates a top surface of a transistor 1200D.For simplification of the figure, some films are omitted in FIG. 24A.FIG. 24B is a cross-sectional view taken along dashed-dotted line X1-X2in FIG. 24A, and FIG. 24C is a cross-sectional view taken alongdashed-dotted line Y1-Y2 in FIG. 24A. Note that the direction of thedashed-dotted line X1-X2 may be called the channel length direction, andthe direction of the dashed-dotted line Y1-Y2 may be called the channelwidth direction.

Note that in the transistor 1200D in FIGS. 24A to 24C, components havingthe same function as the components in the transistor 1200A in FIGS. 21Ato 21C are denoted by the same reference numerals.

In the structure shown in FIGS. 24A to 24C, conductors functioning as asource and a drain have a stacked-layer structure. It is preferable thata conductor which is highly adhesive to the oxide 1230 b be used as theconductors 1240 a and 1240 b, and a material with high conductivity beused as conductors 1241 a and 1241 b. The conductors 1240 a and 1240 bare preferably formed by an ALD method. When the conductors 1240 a and1240 b are formed by an ALD method or the like, coverage can beimproved.

For example, when metal oxide including indium is used as the oxide 1230b, titanium nitride or the like may be used as the conductors 1240 a and1240 b. When a material having high conductivity, such as tantalum,tungsten, copper, or aluminum, is used for the conductors 1241 a and1241 b, the transistor with high reliability and low power consumptioncan be provided.

As illustrated in FIGS. 24B and 24C, the oxide 1230 b is covered withthe conductor 1205 and the conductor 1260 in the channel width directionof the transistor. The insulator 1222 has a projection, whereby the sidesurface of the oxide 1230 b is also covered with the conductor 1260.

Here, when a high-k material such as hafnium oxide is used as theinsulator 1222, the equivalent oxide (SiO₂) thickness (EOT) of theinsulator 1222 can be small because the insulator 1222 has a highrelative dielectric constant. Accordingly, the distance between theconductor 1205 and the oxide 1230 can be increased owing to the physicalthickness of the insulator 1222, without a reduction in the influence ofthe electric field which is applied from the conductor 1205 to the oxide1230. Thus, the distance between the conductor 1205 and the oxide 1230can be adjusted by changing the thickness of the insulator 1222.

For example, at the side surfaces of the oxide 1230 b, the bottomsurface of the conductor 1260 is preferably located closer to thesubstrate than the bottom surface of the oxide 1230 b by adjusting theshape of the projection of the insulator 1224. That is, the transistor1200D has a structure in which the oxide 1230 b can be electricallysurrounded by an electric field of the conductor 1260. Thus, like thetransistor 1200C, the transistor 1200D has an s-channel structure. Inthe transistor 1200D with the s-channel structure, a channel can beformed in the whole (bulk) of the oxide semiconductor 1230 b. In thes-channel structure, the drain current of the transistor is increased,so that a larger amount of on-state current (current which flows betweenthe source and the drain when the transistor is on) can be obtained.Furthermore, the entire channel formation region of the oxide 1230 b canbe depleted by the electric field of the conductor 1260. Accordingly,the off-state current of the s-channel transistor can be furtherreduced. When the channel width is shortened, the effects of thes-channel structure, such as an increase in on-state current and areduction in off-state current, can be enhanced.

<Transistor Structure 5>

FIGS. 25A to 25C illustrate an example of a transistor structuredifferent from those in FIGS. 21A to 21C, FIGS. 22A to 22C, FIGS. 23A to23C, and FIGS. 24A to 24C. FIG. 25A illustrates a top surface of atransistor 1200E. For simplification of the drawing, some films are notillustrated in FIG. 25A. FIG. 25B is a cross-sectional view taken alongdashed-dotted line X1-X2 in FIG. 25A, and FIG. 25C is a cross-sectionalview taken along dashed-dotted line Y1-Y2. Note that the direction ofthe dashed-dotted line X1-X2 may be called the channel length direction,and the direction of the dashed-dotted line Y1-Y2 may be called thechannel width direction.

Note that in the transistor 1200E in FIGS. 25A to 25C, components havingthe same function as the components in the transistor 1200A in FIGS. 21Ato 21C are denoted by the same reference numerals.

In the transistor 1200E illustrated in FIGS. 25A to 25C, the oxide 1230c, the insulator 1250, and the conductor 1260 are formed in an openingformed in the insulator 1280. Furthermore, an end portion of each of theconductor 1240 a and the conductor 1240 b is aligned with an end portionof the opening formed in the insulator 1280. Furthermore, the endportion of each of the conductor 1240 a and the conductor 1240 b isaligned with part of an end portion of the oxide 1230. Thus, theconductors 1240 a and 1240 b can be formed concurrently with the openingin the insulator 1280. This leads to a reduction in the number of masksand steps and improvement in yield and productivity.

The conductor 1240 a, the conductor 1240 b, the oxide 1230 c, and theoxide 1230 d are in contact with the insulator 1280 having theoxygen-excess region with the oxide 1230 d located therebetween. Thus,since the oxide 1230 d exists between the insulator 1280 and the oxide1230 b including the region where the channel is formed, impurities suchas hydrogen, water, and halogen can be prevented from diffusing from theinsulator 1280 into the oxide 1230 b.

Since the transistor 1200E illustrated in FIGS. 25A to 25C has astructure where the conductors 1240 a and 1240 b hardly overlap with theconductor 1260, the parasitic capacitance added to the conductor 1260can be reduced. Thus, the transistor with a high operation frequency canbe provided.

<Transistor Structure 6>

FIGS. 26A to 26C illustrates an example of a transistor structuredifferent from those in FIGS. 21A to 21C, FIGS. 22A to 22C, FIGS. 23A to23C, FIGS. 24A to 24C, and FIGS. 25A to 25C. FIG. 26A illustrates a topsurface of a transistor 1200F. For simplification of the figure, somefilms are not illustrated in FIG. 26A. FIG. 26B is a cross-sectionalview taken along dashed-dotted line X1-X2 in FIG. 26A, and FIG. 26C is across-sectional view taken along dashed-dotted line Y1-Y2 in FIG. 26A.Note that the direction of the dashed-dotted line X1-X2 may be calledthe channel length direction, and the direction of the dashed-dottedline Y1-Y2 may be called the channel width direction.

Note that in the transistor 1200F in FIGS. 26A to 26C, components havingthe same function as the components in the transistor 1200E in FIGS. 25Ato 25C are denoted by the same reference numerals.

An insulator 1285 and an insulator 1286 are formed over the insulator1282.

In an opening formed in the insulator 1280, the insulator 1282, and theinsulator 1285, the oxide 1230 c, the insulator 1250, and the conductor1260 are formed. Furthermore, an end portion of each of the conductor1240 a and the conductor 1240 b is aligned with an end portion of theopening formed in the insulator 1280. Furthermore, the end portion ofeach of the conductor 1240 a and the conductor 1240 b is aligned withpart of end portions of the oxide 1230 c. Thus, the conductors 1240 aand 1240 b can be formed concurrently with the opening in the insulator1280. This leads to a reduction in the number of masks and steps andimprovement in yield and productivity.

The conductor 1240 a, the conductor 1240 b, the oxide 1230 c, and theoxide 1230 d are in contact with the insulator 1280 having theoxygen-excess region with the oxide 1230 d located therebetween. Thus,since the oxide 1230 d exists between the insulator 1280 and the oxide1230 b including the region where the channel is formed, impurities suchas hydrogen, water, and halogen can be prevented from diffusing from theinsulator 1280 into the oxide 1230 b.

In addition, since a high-resistance offset region is not formed in thetransistor 1200F illustrated in FIGS. 26A to 26C, the on-state currentof the transistor can be increased.

<Transistor Structure 7>

FIGS. 27A to 27C illustrate an example of a transistor structuredifferent from those in FIGS. 21A to 21C, FIGS. 22A to 22C, FIGS. 23A to23C, FIGS. 24A to 24C, FIGS. 25A to 25C, and FIGS. 26A to 26C. FIG. 27Aillustrates a top surface of a transistor 1200G. For simplification ofthe figure, some films are omitted in FIG. 27A. FIG. 27B is across-sectional view taken along dashed-dotted line X1-X2 in FIG. 27A,and FIG. 27C is a cross-sectional view taken along dashed-dotted lineY1-Y2 in FIG. 27A. Note that the direction of the dashed-dotted lineX1-X2 may be called the channel length direction, and the direction ofthe dashed-dotted line Y1-Y2 may be called the channel width direction.

Note that in the transistor 1200G in FIGS. 27A to 27C, components havingthe same function as the components in the transistor 1200A in FIGS. 21Ato 21C are denoted by the same reference numerals.

The transistor 1200G illustrated in FIGS. 27A to 27C does not have theoxide 1230 d. For example, when the conductor 1240 a and the conductor1240 b are formed using a conductor with a high oxidation resistance,the oxide 1230 d is not necessarily provided. This leads to a reductionin the number of masks and steps and improvement in yield andproductivity.

The insulator 1224 may be provided in only the region overlapping withthe oxide 1230 a and the oxide 1230 b. In that case, the oxide 1230 a,the oxide 1230 b, and the insulator 1224 can be processed using theinsulator 1222 as an etching stopper. As a result, yield andproductivity can be improved.

Since the transistor 1200G illustrated in FIGS. 27A to 27C has astructure in which the conductors 1240 a and 1240 b hardly overlap withthe conductor 1260, the parasitic capacitance added to the conductor1260 can be reduced. Thus, the transistor with a high operationfrequency can be provided.

Note that this embodiment can be combined with any of the otherembodiments in this specification as appropriate.

Embodiment 6

In this embodiment, a structure of an oxide semiconductor film that canbe used for the oxide 1230 described in the above embodiment will bedescribed.

An oxide semiconductor is classified into a single crystal oxidesemiconductor and a non-single-crystal oxide semiconductor. Examples ofa non-single-crystal oxide semiconductor include a c-axis-alignedcrystalline oxide semiconductor (CAAC-OS), a polycrystalline oxidesemiconductor, a nanocrystalline oxide semiconductor (nc-OS), anamorphous-like oxide semiconductor (a-like OS), and an amorphous oxidesemiconductor.

From another perspective, an oxide semiconductor is classified into anamorphous oxide semiconductor and a crystalline oxide semiconductor.Examples of a crystalline oxide semiconductor include a single crystaloxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor,and an nc-OS.

An amorphous structure is generally thought to be isotropic and have nonon-uniform structure, to be metastable and not to have fixed positionsof atoms, to have a flexible bond angle, and to have a short-range orderbut have no long-range order, for example.

This means that a stable oxide semiconductor cannot be regarded as acompletely amorphous oxide semiconductor. Moreover, an oxidesemiconductor that is not isotropic (e.g., an oxide semiconductor thathas a periodic structure in a microscopic region) cannot be regarded asa completely amorphous oxide semiconductor. In contrast, an a-like OS,which is not isotropic, has an unstable structure that contains a void.Because of its instability, an a-like OS is close to an amorphous oxidesemiconductor in terms of physical properties.

<CAAC-OS>

First, a CAAC-OS will be described.

A CAAC-OS is one of oxide semiconductors having a plurality of c-axisaligned crystal parts (also referred to as pellets).

Analysis of a CAAC-OS by X-ray diffraction (XRD) is described. Forexample, when the structure of a CAAC-OS including an InGaZnO₄ crystalthat is classified into the space group R-3m is analyzed by anout-of-plane method, a peak appears at a diffraction angle (2θ) ofaround 31° as shown in FIG. 31A. This peak is derived from the (009)plane of the InGaZnO₄ crystal, which indicates that crystals in theCAAC-OS have c-axis alignment, and that the c-axes are aligned in adirection substantially perpendicular to a surface over which theCAAC-OS film is formed (also referred to as a formation surface) or thetop surface of the CAAC-OS film. Note that a peak sometimes appears at a2θ of around 36° in addition to the peak at a 2θ of around 31°. The peakat a 2θ of around 36° is derived from a crystal structure classifiedinto the space group Fd-3m. Therefore, it is preferred that the CAAC-OSdo not show the peak at a 2θ of around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-planemethod in which an X-ray is incident on the CAAC-OS in a directionparallel to the formation surface, a peak appears at a 2θ of around 56°.This peak is derived from the (110) plane of the InGaZnO₄ crystal. Whenanalysis (q scan) is performed with 2θ fixed at around 56° and with thesample rotated using a normal vector to the sample surface as an axis (ϕaxis), as shown in FIG. 31B, a peak is not clearly observed. Incontrast, in the case where single crystal InGaZnO₄ is subjected to ϕscan with 2θ fixed at around 56°, as shown in FIG. 31C, six peaks whichare derived from crystal planes equivalent to the (110) plane areobserved. Accordingly, the structural analysis using XRD shows that thedirections of a-axes and b-axes are irregularly oriented in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction will be described. Forexample, when an electron beam with a probe diameter of 300 nm isincident on a CAAC-OS including an InGaZnO₄ crystal in a directionparallel to the formation surface of the CAAC-OS, a diffraction pattern(also referred to as a selected-area electron diffraction pattern) shownin FIG. 31D can be obtained. In this diffraction pattern, spots derivedfrom the (009) plane of an InGaZnO₄ crystal are included. Thus, theelectron diffraction also indicates that pellets included in the CAAC-OShave c-axis alignment and that the c-axes are aligned in a directionsubstantially perpendicular to the formation surface or the top surfaceof the CAAC-OS. Meanwhile, FIG. 31E shows a diffraction pattern obtainedin such a manner that an electron beam with a probe diameter of 300 nmis incident on the same sample in a direction perpendicular to thesample surface. As shown in FIG. 31E, a ring-like diffraction pattern isobserved. Thus, the electron diffraction using an electron beam with aprobe diameter of 300 nm also indicates that the a-axes and b-axes ofthe pellets included in the CAAC-OS do not have regular orientation. Thefirst ring in FIG. 31E is considered to be derived from the (010) plane,the (100) plane, and the like of the InGaZnO₄ crystal. The second ringin FIG. 31E is considered to be derived from the (110) plane and thelike.

In a combined analysis image (also referred to as a high-resolution TEMimage) of a bright-field image and a diffraction pattern of a CAAC-OS,which is obtained using a transmission electron microscope (TEM), aplurality of pellets can be observed. However, even in thehigh-resolution TEM image, a boundary between pellets, that is, acrystal grain boundary is not clearly observed in some cases. Thus, inthe CAAC-OS, a reduction in electron mobility due to the grain boundaryis less likely to occur.

FIG. 32A shows a high-resolution TEM image of a cross section of theCAAC-OS which is observed from a direction substantially parallel to thesample surface. The high-resolution TEM image is obtained with aspherical aberration corrector function. The high-resolution TEM imageobtained with a spherical aberration corrector function is particularlyreferred to as a Cs-corrected high-resolution TEM image. TheCs-corrected high-resolution TEM image can be observed with, forexample, an atomic resolution analytical electron microscope JEM-ARM200Fmanufactured by JEOL Ltd.

FIG. 32A shows pellets in which metal atoms are arranged in a layeredmanner. FIG. 32A proves that the size of a pellet is greater than orequal to 1 nm or greater than or equal to 3 nm. Therefore, the pelletcan also be referred to as a nanocrystal (nc). Furthermore, the CAAC-OScan also be referred to as an oxide semiconductor including c-axisaligned nanocrystals (CANC). A pellet reflects unevenness of a formationsurface or a top surface of the CAAC-OS, and is parallel to theformation surface or the top surface of the CAAC-OS.

FIGS. 32B and 32C show Cs-corrected high-resolution TEM images of aplane of the CAAC-OS observed from a direction substantiallyperpendicular to the sample surface. FIGS. 32D and 32E are imagesobtained through image processing of FIGS. 32B and 32C. The method ofimage processing is as follows. The image in FIG. 32B is subjected tofast Fourier transform (FFT), so that an FFT image is obtained. Then,mask processing is performed such that a range of from 2.8 nm⁻¹ to 5.0nm⁻¹ from the origin in the obtained FFT image remains. After the maskprocessing, the FFT image is processed by inverse fast Fourier transform(IFFT) to obtain a processed image. The image obtained in this manner iscalled an FFT filtering image. The FFT filtering image is a Cs-correctedhigh-resolution TEM image from which a periodic component is extracted,and shows a lattice arrangement.

In FIG. 32D, a portion where a lattice arrangement is broken is denotedwith a dashed line. A region surrounded by a dashed line is one pellet.The portion denoted with the dashed line is a junction of pellets. Thedashed line draws a hexagon, which means that the pellet has a hexagonalshape. Note that the shape of the pellet is not always a regular hexagonbut is a non-regular hexagon in many cases.

In FIG. 32E, a dotted line denotes a portion between a region where alattice arrangement is well aligned and another region where a latticearrangement is well aligned, and a dashed line denotes the direction ofthe lattice arrangement. A clear crystal grain boundary cannot beobserved even in the vicinity of the dotted line. When a lattice pointin the vicinity of the dotted line is regarded as a center andsurrounding lattice points are joined, a distorted hexagon, pentagon,and/or heptagon can be formed, for example. That is, a latticearrangement is distorted so that formation of a crystal grain boundaryis inhibited. This is probably because the CAAC-OS can toleratedistortion owing to a low density of arrangement of oxygen atoms in thea-b plane direction, an interatomic bond distance changed bysubstitution of a metal element, and the like.

As described above, the CAAC-OS has c-axis alignment, its pellets(nanocrystals) are connected in an a-b plane direction, and the crystalstructure has distortion. For this reason, the CAAC-OS can also bereferred to as an oxide semiconductor including a c-axis-aligneda-b-plane-anchored (CAA) crystal.

The CAAC-OS is an oxide semiconductor with high crystallinity. Entry ofimpurities, formation of defects, or the like might decrease thecrystallinity of an oxide semiconductor. This means that the CAAC-OS hassmall amounts of impurities and defects (e.g., oxygen vacancies).

Note that the impurity means an element other than the main componentsof the oxide semiconductor, such as hydrogen, carbon, silicon, or atransition metal element. For example, an element (specifically, siliconor the like) having higher strength of bonding to oxygen than a metalelement included in an oxide semiconductor extracts oxygen from theoxide semiconductor, which results in disorder of the atomic arrangementand reduced crystallinity of the oxide semiconductor. A heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disturbs the atomic arrangementof the oxide semiconductor and decreases crystallinity.

<nc-OS>

Next, an nc-OS is described.

Analysis of an nc-OS by XRD is described. When the structure of an nc-OSis analyzed by an out-of-plane method, a peak indicating orientationdoes not appear. That is, a crystal of an nc-OS does not haveorientation.

For example, when an electron beam with a probe diameter of 50 nm isincident on a 34-nm-thick region of thinned nc-OS including an InGaZnO₄crystal in a direction parallel to the formation surface, a ring-shapeddiffraction pattern (a nanobeam electron diffraction pattern) shown inFIG. 33A is observed. FIG. 33B shows a diffraction pattern obtained whenan electron beam with a probe diameter of 1 nm is incident on the samesample. As shown in FIG. 33B, a plurality of spots are observed in aring-like region. In other words, ordering in an nc-OS is not observedwith an electron beam with a probe diameter of 50 nm but is observedwith an electron beam with a probe diameter of 1 nm.

Furthermore, an electron diffraction pattern in which spots are arrangedin an approximately regular hexagonal shape is observed in some cases asshown in FIG. 33C when an electron beam having a probe diameter of 1 nmis incident on a region with a thickness of less than 10 nm. This meansthat an nc-OS has a well-ordered region, i.e., a crystal, in the rangeof less than 10 nm in thickness. Note that an electron diffractionpattern having regularity is not observed in some regions becausecrystals are aligned in various directions.

FIG. 33D shows a Cs-corrected high-resolution TEM image of a crosssection of an nc-OS observed from the direction substantially parallelto the formation surface. In a high-resolution TEM image, an nc-OS has aregion in which a crystal part is observed, such as the part indicatedby additional lines in FIG. 33D, and a region in which a crystal part isnot clearly observed. In most cases, the size of a crystal part includedin the nc-OS is greater than or equal to 1 nm and less than or equal to10 nm, or specifically, greater than or equal to 1 nm and less than orequal to 3 nm. Note that an oxide semiconductor including a crystal partwhose size is greater than 10 nm and less than or equal to 100 nm issometimes referred to as a microcrystalline oxide semiconductor. In ahigh-resolution TEM image of the nc-OS, for example, a grain boundary isnot clearly observed in some cases. Note that there is a possibilitythat the origin of the nanocrystal is the same as that of a pellet in aCAAC-OS. Therefore, a crystal part of the nc-OS may be referred to as apellet in the following description.

As described above, in the nc-OS, a microscopic region (for example, aregion with a size greater than or equal to 1 nm and less than or equalto 10 nm, in particular, a region with a size greater than or equal to 1nm and less than or equal to 3 nm) has a periodic atomic arrangement.There is no regularity of crystal orientation between different pelletsin the nc-OS. Thus, the orientation of the whole film is not observed.Accordingly, the nc-OS cannot be distinguished from an a-like OS or anamorphous oxide semiconductor, depending on an analysis method.

Since there is no regularity of crystal orientation between the pellets(nanocrystals) as mentioned above, the nc-OS can also be referred to asan oxide semiconductor including random aligned nanocrystals (RANC) oran oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has high regularity as comparedwith an amorphous oxide semiconductor. Therefore, the nc-OS is likely tohave a lower density of defect states than an a-like OS and an amorphousoxide semiconductor. Note that there is no regularity of crystalorientation between different pellets in the nc-OS. Therefore, the nc-OShas a higher density of defect states than the CAAC-OS.

<A-Like OS>

An a-like OS has a structure between those of the nc-OS and theamorphous oxide semiconductor.

FIGS. 34A and 34B are high-resolution cross-sectional TEM images of ana-like OS. FIG. 34A is the high-resolution cross-sectional TEM image ofthe a-like OS at the start of the electron irradiation. FIG. 34B is thehigh-resolution cross-sectional TEM image of the a-like OS after theelectron (e⁻) irradiation at 4.3×10⁸ e⁻/nm². FIGS. 34A and 34B show thatstripe-like bright regions extending vertically are observed in thea-like OS from the start of the electron irradiation. It can also befound that the shape of the bright region changes after the electronirradiation. Note that the bright region is presumably a void or alow-density region.

The a-like OS has an unstable structure because it contains a void. Toverify that an a-like OS has an unstable structure as compared with aCAAC-OS and an nc-OS, a change in structure caused by electronirradiation is described below.

An a-like OS, an nc-OS, and a CAAC-OS are prepared as samples. Each ofthe samples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample isobtained. The high-resolution cross-sectional TEM images show that allthe samples have crystal parts.

It is known that a unit cell of an InGaZnO₄ crystal has a structure inwhich nine layers including three In—O layers and six Ga—Zn—O layers arestacked in the c-axis direction. The distance between the adjacentlayers is equivalent to the lattice spacing on the (009) plane (alsoreferred to as d value). The value is calculated to be 0.29 nm fromcrystal structural analysis. Accordingly, a portion where the spacingbetween lattice fringes is greater than or equal to 0.28 nm and lessthan or equal to 0.30 nm is regarded as a crystal part of InGaZnO₄ inthe following description. Each of lattice fringes corresponds to thea-b plane of the InGaZnO₄ crystal.

FIG. 35 shows change in the average size of crystal parts (at 22 pointsto 30 points) in each sample. Note that the crystal part sizecorresponds to the length of a lattice fringe. FIG. 35 indicates thatthe crystal part size in the a-like OS increases with an increase in thecumulative electron dose in obtaining TEM images, for example. As shownin FIG. 35, a crystal part of approximately 1.2 nm (also referred to asan initial nucleus) at the start of TEM observation grows to a size ofapproximately 1.9 nm at a cumulative electron (e⁻) dose of 4.2×10⁸e⁻/nm². In contrast, the crystal part size in the nc-OS and the CAAC-OSshows little change from the start of electron irradiation to acumulative electron dose of 4.2×10⁸ e⁻/nm². As shown in FIG. 35, thecrystal part sizes in an nc-OS and a CAAC-OS are approximately 1.3 nmand approximately 1.8 nm, respectively, regardless of the cumulativeelectron dose. For the electron beam irradiation and TEM observation, aHitachi H-9000NAR transmission electron microscope was used. Theconditions of electron beam irradiation were as follows: theaccelerating voltage was 300 kV; the current density was 6.7×10⁵e⁻/(nm²·s); and the diameter of the irradiation region was 230 nm.

In this manner, growth of the crystal part in the a-like OS is inducedby electron irradiation. In contrast, in the nc-OS and the CAAC-OS,growth of the crystal part is hardly induced by electron irradiation.Therefore, the a-like OS has an unstable structure as compared with thenc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS becauseit contains a void. Specifically, the density of the a-like OS is higherthan or equal to 78.6% and lower than 92.3% of the density of the singlecrystal oxide semiconductor having the same composition. The density ofeach of the nc-OS and the CAAC-OS is higher than or equal to 92.3% andlower than 100% of the density of the single crystal oxide semiconductorhaving the same composition. Note that it is difficult to deposit anoxide semiconductor having a density of lower than 78% of the density ofthe single crystal oxide semiconductor.

For example, in the case of an oxide semiconductor having an atomicratio of In:Ga:Zn=1:1:1, the density of single crystal InGaZnO₄ with arhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the caseof the oxide semiconductor having an atomic ratio of In:Ga:Zn=1:1:1, thedensity of the a-like OS is higher than or equal to 5.0 g/cm³ and lowerthan 5.9 g/cm³. For example, in the case of the oxide semiconductorhaving an atomic ratio of In:Ga:Zn=1:1:1, the density of each of thenc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lowerthan 6.3 g/cm³.

Note that in the case where an oxide semiconductor having a certaincomposition does not exist in a single crystal structure, single crystaloxide semiconductors with different compositions are combined at anadequate ratio, which makes it possible to calculate density equivalentto that of a single crystal oxide semiconductor with the desiredcomposition. The density of a single crystal oxide semiconductor havingthe desired composition can be estimated using a weighted averageaccording to the combination ratio of the single crystal oxidesemiconductors with different compositions. Note that it is preferableto use as few kinds of single crystal oxide semiconductors as possibleto estimate the density.

As described above, oxide semiconductors have various structures andvarious properties. Note that an oxide semiconductor may be a stackedlayer including two or more films of an amorphous oxide semiconductor,an a-like OS, an nc-OS, and a CAAC-OS, for example.

<Carrier Density of Oxide Semiconductor>

Next, the carrier density of an oxide semiconductor will be describedbelow.

Examples of a factor affecting the carrier density of an oxidesemiconductor include oxygen vacancy (Vo) and impurities in the oxidesemiconductor.

As the amount of oxygen vacancy in the oxide semiconductor increases,the density of defect states increases when hydrogen is bonded to theoxygen vacancy (this state is also referred to as VoH). The density ofdefect states also increases with an increase in the amount of impurityin the oxide semiconductor. Hence, the carrier density of an oxidesemiconductor can be controlled by controlling the density of defectstates in the oxide semiconductor.

A transistor using the oxide semiconductor in a channel region will bedescribed below.

The carrier density of the oxide semiconductor is preferably reduced inorder to inhibit the negative shift of the threshold voltage of thetransistor or reduce the off-state current of the transistor. In orderto reduce the carrier density of the oxide semiconductor, the impurityconcentration in the oxide semiconductor is reduced so that the densityof defect states can be reduced. In this specification and the like, astate with a low impurity concentration and a low density of defectstates is referred to as a highly purified intrinsic or substantiallyhighly purified intrinsic state. The carrier density of a highlypurified intrinsic oxide semiconductor is lower than 8×10¹⁵ cm⁻³,preferably lower than 1×10¹¹ cm⁻³, and further preferably lower than1×10¹⁰ cm⁻³ and is higher than or equal to 1×10⁹ cm⁻³.

In contrast, the carrier density of the oxide semiconductor ispreferably increased in order to improve the on-state current of thetransistor or improve the field-effect mobility of the transistor. Inorder to increase the carrier density of the oxide semiconductor, theimpurity concentration or the density of defect states in the oxidesemiconductor is slightly increased. Alternatively, the bandgap of theoxide semiconductor is preferably narrowed. For example, an oxidesemiconductor that has a slightly high impurity concentration or aslightly high density of defect states in the range where a favorableon/off ratio is obtained in the I_(d)—V_(g) characteristics of thetransistor can be regarded as substantially intrinsic. Furthermore, anoxide semiconductor that has a high electron affinity and thus has anarrow bandgap so as to increase the density of thermally excitedelectrons (carriers) can be regarded as substantially intrinsic. Notethat a transistor using an oxide semiconductor with higher electronaffinity has lower threshold voltage.

The aforementioned oxide semiconductor with an increased carrier densityhas somewhat n-type conductivity; thus, it can be referred to as a“slightly-n” oxide semiconductor.

The carrier density of a substantially intrinsic oxide semiconductor ispreferably higher than or equal to 1×10⁵ cm⁻³ and lower than 1×10¹⁸cm⁻³, further preferably higher than or equal to 1×10⁷ cm⁻³ and lowerthan or equal to 1×10¹⁷ cm⁻³, still further preferably higher than orequal to 1×10⁹ cm⁻³ and lower than or equal to 5×10¹⁶ cm⁻³, yet furtherpreferably higher than or equal to 1×10¹⁰ cm⁻³ and lower than or equalto 1×10¹⁶ cm⁻³, and yet still preferably higher than or equal to 1×10¹¹cm⁻³ and lower than or equal to 1×10¹⁵ cm⁻³.

Note that this embodiment can be combined with any of the otherembodiments in this specification as appropriate.

(Notes on the Description in this Specification and the Like)

The following are notes on the structures in the above embodiments.

<Notes on One Embodiment of the Present Invention Described inEmbodiments>

One embodiment of the present invention can be constituted byappropriately combining the structure described in an embodiment withany of the structures described in the other embodiments. In addition,in the case where a plurality of structure examples are described in oneembodiment, some of the structure examples can be combined asappropriate.

Note that what is described (or part thereof) in an embodiment can beapplied to, combined with, or replaced with another content in the sameembodiment and/or what is described (or part thereof) in anotherembodiment or other embodiments.

Note that in each embodiment, a content described in the embodiment is acontent described with reference to a variety of diagrams or a contentdescribed with text disclosed in this specification.

Note that by combining a diagram (or part thereof) described in oneembodiment with another part of the diagram, a different diagram (orpart thereof) described in the embodiment, and/or a diagram (or partthereof) described in another embodiment or other embodiments, much morediagrams can be formed.

<Notes on Ordinal Numbers>

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents. Thus, the terms do not limit the number or order ofcomponents. In the present specification and the like, a “first”component in one embodiment can be referred to as a “second” componentin other embodiments or claims. Furthermore, in the presentspecification and the like, a “first” component in one embodiment can bereferred to without the ordinal number in other embodiments or claims.

<Notes on the Description for Drawings>

Embodiments are described with reference to drawings. However, theembodiments can be implemented with various modes. It will be readilyappreciated by those skilled in the art that modes and details can bechanged in various ways without departing from the spirit and scope ofthe present invention. Thus, the present invention should not beinterpreted as being limited to the description of the embodiments. Notethat in the structures of the embodiments, the same portions or portionshaving similar functions are denoted by the same reference numerals indifferent drawings, and the description of such portions is notrepeated.

In this specification and the like, the terms for explainingarrangement, such as “over” and “under”, are used for convenience todescribe the positional relation between components with reference todrawings. Furthermore, the positional relation between components ischanged as appropriate in accordance with a direction in which thecomponents are described. Therefore, the terms for explainingarrangement are not limited to those used in this specification and maybe changed to other terms as appropriate depending on the situation.

The term “over” or “under” does not necessarily mean that a component isplaced directly over or directly under and directly in contact withanother component. For example, the expression “electrode B overinsulating layer A” does not necessarily mean that the electrode B is onand in direct contact with the insulating layer A and can mean the casewhere another component is provided between the insulating layer A andthe electrode B.

Furthermore, in a block diagram in this specification and the like,components are functionally classified and shown by blocks that areindependent from each other. However, in an actual circuit and the like,such components are sometimes hard to classify functionally, and thereis a case in which one circuit is concerned with a plurality offunctions or a case in which a plurality of circuits are concerned withone function. Therefore, blocks in a block diagram do not necessarilyshow components described in the specification, which can be explainedwith another term as appropriate depending on the situation.

In drawings, the size, the layer thickness, or the region is determinedarbitrarily for description convenience. Therefore, the size, the layerthickness, or the region is not limited to the illustrated scale. Notethat the drawings are schematically shown for clarity, and embodimentsof the present invention are not limited to shapes or values shown inthe drawings. For example, the following can be included: variation insignal, voltage, or current due to noise or difference in timing.

In drawings such as a top view (also referred to as a plan view or alayout view) and a perspective view, some of components might not beillustrated for clarity of the drawings.

In the drawings, the same components, components having similarfunctions, components formed of the same material, or components formedat the same time are denoted by the same reference numerals in somecases, and the description thereof is not repeated in some cases.

<Notes on Expressions that can be Rephrased>

In this specification or the like, the terms “one of a source and adrain” (or a first electrode or a first terminal) and “the other of thesource and the drain” (or a second electrode or a second terminal) areused to describe the connection relation of a transistor. This isbecause a source and a drain of a transistor are interchangeabledepending on the structure, operation conditions, or the like of thetransistor. Note that the source or the drain of the transistor can alsobe referred to as a source (or drain) terminal, a source (or drain)electrode, or the like as appropriate depending on the situation. Inthis specification and the like, two terminals except a gate aresometimes referred to as a first terminal and a second terminal or as athird terminal and a fourth terminal. In this specification and thelike, in the case where a transistor has two or more gates (such astructure is referred to as a dual-gate structure in some cases), thesegates are referred to as a first gate and a second gate or a front gateand a back gate in some cases. Note that a “bottom gate” is a terminalwhich is formed before a channel formation region in manufacture of atransistor, and a “top gate” is a terminal which is formed after achannel formation region in manufacture of a transistor.

A transistor is an element having three terminals: a gate, a source, anda drain. A gate is a terminal which functions as a control terminal forcontrolling the conduction state of a transistor. Functions ofinput/output terminals of the transistor depend on the type and thelevels of potentials applied to the terminals, and one of the twoterminals serves as a source and the other serves as a drain. Therefore,the terms “source” and “drain” can be switched in this specification andthe like. In this specification and the like, two terminals except agate are sometimes referred to as a first terminal and a second terminalor as a third terminal and a fourth terminal.

In addition, in this specification and the like, the term such as an“electrode” or a “wiring” does not limit a function of the component.For example, an “electrode” is used as part of a “wiring” in some cases,and vice versa. Furthermore, the term “electrode” or “wiring” can alsomean a combination of a plurality of “electrodes” and “wirings” formedin an integrated manner.

In this specification and the like, “voltage” and “potential” can bereplaced with each other. The term “voltage” refers to a potentialdifference from a reference potential. When the reference potential is aground potential, for example, “voltage” can be replaced with“potential.” The ground potential does not necessarily mean 0 V.Potentials are relative values, and the potential applied to a wiring orthe like is changed depending on the reference potential, in some cases.Note that in this specification, the reference potential refers to areference potential only in a certain portion or place, and does notrefer to a reference potential in another portion or place.

In this specification and the like, the terms “film” and “layer” can beinterchanged with each other depending on the case or circumstances. Forexample, the term “conductive layer” can be changed into the term“conductive film” in some cases. Moreover, the term “insulating film”can be changed into the term “insulating layer” in some cases, or can bereplaced with a word not including the term “film” or “layer” dependingon the case or circumstances. For example, the term “conductive layer”or “conductive film” can be changed into the term “conductor” in somecases. Furthermore, for example, the term “insulating layer” or“insulating film” can be changed into the term “insulator” in somecases.

In this specification and the like, the terms “wiring”, “signal line”,“power supply line”, and the like can be interchanged with each otherdepending on circumstances or conditions. For example, the term “wiring”can be changed into the term “signal line” in some cases. For example,the term “wiring” can be changed into the term such as “power sourceline” in some cases. The term such as “signal line” or “power sourceline” can be changed into the term “wiring” in some cases. The term suchas “power source line” can be changed into the term such as “signalline” in some cases. The term such as “signal line” can be changed intothe term such as “power source line” in some cases. The term “potential”that is applied to a wiring can be changed into the term “signal” or thelike depending on circumstances or conditions. Inversely, the term“signal” or the like can be changed into the term “potential” in somecases.

<Notes on Definitions of Terms>

The following are definitions of the terms mentioned in the aboveembodiments.

«Semiconductor»

In this specification, a “semiconductor” may have characteristics of an“insulator” in some cases when the conductivity is sufficiently low, forexample. Furthermore, a “semiconductor” and an “insulator” cannot bestrictly distinguished from each other in some cases because a borderbetween the “semiconductor” and the “insulator” is not clear.Accordingly, a “semiconductor” in this specification can be called an“insulator” in some cases. Similarly, an “insulator” in thisspecification can be called a “semiconductor” in some cases.

Furthermore, a “semiconductor” includes characteristics of a “conductor”in some cases when the conductivity is sufficiently high, for example.Furthermore, a “semiconductor” and a “conductor” cannot be strictlydistinguished from each other in some cases because a border between the“semiconductor” and the “conductor” is not clear. Accordingly, a“semiconductor” in this specification can be called a “conductor” insome cases. Similarly, a “conductor” in this specification can be calleda “semiconductor” in some cases.

Note that an impurity in a semiconductor refers to, for example,elements other than the main components of a semiconductor layer. Forexample, an element with a concentration of lower than 0.1 atomic % isan impurity. When an impurity is contained, the density of states (DOS)may be formed in a semiconductor, the carrier mobility may be decreased,or the crystallinity may be decreased. In the case where thesemiconductor is an oxide semiconductor, examples of an impurity whichchanges characteristics of the semiconductor include Group 1 elements,Group 2 elements, Group 13 elements, Group 14 elements, Group 15elements, and transition metals other than the main components of thesemiconductor; specifically, there are hydrogen (included in water),lithium, sodium, silicon, boron, phosphorus, carbon, and nitrogen, forexample. When the semiconductor is an oxide semiconductor, oxygenvacancies may be formed by entry of impurities such as hydrogen, forexample. Furthermore, when the semiconductor is a silicon layer,examples of an impurity which changes the characteristics of thesemiconductor include oxygen, Group 1 elements except hydrogen, Group 2elements, Group 13 elements, and Group 15 elements.

«Transistor»

In this specification, a transistor is an element having at least threeterminals of a gate, a drain, and a source. The transistor has a channelformation region between the drain (a drain terminal, a drain region, ora drain electrode) and the source (a source terminal, a source region,or a source electrode), and current can flow through the channelformation region between the source and the drain. Note that in thisspecification and the like, a channel formation region refers to aregion through which current mainly flows.

Furthermore, functions of a source and a drain might be switched whentransistors having different polarities are employed or a direction ofcurrent flow is changed in circuit operation, for example. Therefore,the terms “source” and “drain” can be switched in this specification andthe like.

«Switch»

In this specification and the like, a switch is conducting (on state) ornot conducting (off state) to determine whether current flowstherethrough or not. Alternatively, a switch has a function of selectingand changing a current path.

Examples of a switch include an electrical switch and a mechanicalswitch. That is, any element can be used as a switch as long as it cancontrol current, without limitation to a certain element.

Examples of the electrical switch include a transistor (e.g., a bipolartransistor or a MOS transistor), a diode (e.g., a PN diode, a PIN diode,a Schottky diode, a metal-insulator-metal (MIM) diode, ametal-insulator-semiconductor (MIS) diode, or a diode-connectedtransistor), and a logic circuit in which such elements are combined.

In the case of using a transistor as a switch, an “on state” of thetransistor refers to a state in which a source electrode and a drainelectrode of the transistor are electrically short-circuited.Furthermore, an “off state” of the transistor refers to a state in whichthe source electrode and the drain electrode of the transistor areelectrically cut off. In the case where a transistor operates just as aswitch, the polarity (conductivity type) of the transistor is notparticularly limited to a certain type.

An example of a mechanical switch is a switch formed using a microelectro mechanical systems (MEMS) technology, such as a digitalmicromirror device (DMD). Such a switch includes an electrode which canbe moved mechanically, and operates by controlling conduction andnon-conduction in accordance with movement of the electrode.

«Channel Length»

In this specification and the like, the channel length refers to, forexample, the distance between a source (source region or sourceelectrode) and a drain (drain region or drain electrode) in a regionwhere a semiconductor (or a portion where current flows in asemiconductor when a transistor is on) and a gate electrode overlap witheach other or a region where a channel is formed in a top view of thetransistor.

In one transistor, channel lengths in all regions are not necessarilythe same. In other words, the channel length of one transistor is notfixed to one value in some cases. Therefore, in this specification, thechannel length is any one of values, the maximum value, the minimumvalue, or the average value in a region where a channel is formed.

«Channel Width»

In this specification and the like, the channel width refers to, forexample, the length of a portion where a source and a drain face eachother in a region where a semiconductor (or a portion where a currentflows in a semiconductor when a transistor is on) and a gate electrodeoverlap with each other, or a region where a channel is formed in a topview of the transistor.

In one transistor, channel widths in all regions are not necessarily thesame. In other words, the channel width of one transistor is not fixedto one value in some cases. Therefore, in this specification, a channelwidth is any one of values, the maximum value, the minimum value, or theaverage value in a region where a channel is formed.

Note that depending on transistor structures, a channel width in aregion where a channel is formed actually (hereinafter referred to as aneffective channel width) is different from a channel width shown in atop view of the transistor (hereinafter referred to as an apparentchannel width) in some cases. For example, in a transistor having athree-dimensional structure, an effective channel width is greater thanan apparent channel width shown in a top view of the transistor, and itsinfluence cannot be ignored in some cases. For example, in aminiaturized transistor having a three-dimensional structure, theproportion of a channel region formed in a side surface of asemiconductor is high in some cases. In that case, an effective channelwidth obtained when a channel is actually formed is greater than anapparent channel width shown in the top view.

In a transistor having a three-dimensional structure, an effectivechannel width is difficult to measure in some cases. For example, toestimate an effective channel width from a design value, it is necessaryto assume that the shape of a semiconductor is known as an assumptioncondition. Therefore, in the case where the shape of a semiconductor isnot known accurately, it is difficult to measure an effective channelwidth accurately.

Therefore, in this specification, in a top view of a transistor, anapparent channel width that is a length of a portion where a source anda drain face each other in a region where a semiconductor and a gateelectrode overlap with each other is referred to as a surrounded channelwidth (SCW) in some cases. Furthermore, in this specification, in thecase where the term “channel width” is simply used, it may denote asurrounded channel width and an apparent channel width. Alternatively,in this specification, in the case where the term “channel width” issimply used, it may denote an effective channel width in some cases.Note that the values of a channel length, a channel width, an effectivechannel width, an apparent channel width, a surrounded channel width,and the like can be determined by obtaining and analyzing across-sectional TEM image and the like.

Note that in the case where field-effect mobility, a current value perchannel width, and the like of a transistor are obtained by calculation,a surrounded channel width may be used for the calculation. In thatcase, a value different from one in the case where an effective channelwidth is used for the calculation is obtained in some cases.

«Connection»

In this specification and the like, when it is described that X and Yare connected, the case where X and Y are electrically connected, thecase where X and Y are functionally connected, and the case where X andY are directly connected are included therein. Accordingly, anotherelement may be interposed between elements having a connection relationshown in drawings and texts, without limiting to a predeterminedconnection relation, for example, the connection relation shown in thedrawings and the texts.

Here, X, Y, and the like each denote an object (e.g., a device, anelement, a circuit, a wiring, an electrode, a terminal, a conductivefilm, a layer, or the like).

For example, in the case where X and Y are electrically connected, oneor more elements that enable an electrical connection between X and Y(e.g., a switch, a transistor, a capacitor, an inductor, a resistor, adiode, a display element, a light-emitting element, or a load) can beconnected between X and Y. Note that the switch is controlled to beturned on or off. That is, a switch is conducting or not conducting (isturned on or off) to determine whether current flows therethrough ornot.

For example, in the case where X and Y are functionally connected, oneor more circuits that enable functional connection between X and Y(e.g., a logic circuit such as an inverter, a NAND circuit, or a NORcircuit; a signal converter circuit such as a DA converter circuit, anAD converter circuit, or a gamma correction circuit; a potential levelconverter circuit such as a power source circuit (e.g., a step-upconverter or a step-down converter) or a level shifter circuit forchanging the potential level of a signal; a voltage source; a currentsource; a switching circuit; an amplifier circuit such as a circuit thatcan increase signal amplitude, the amount of current, or the like, anoperational amplifier, a differential amplifier circuit, a sourcefollower circuit, or a buffer circuit; a signal generation circuit; amemory circuit; and/or a control circuit) can be connected between X andY. For example, even when another circuit is interposed between X and Y,X and Y are functionally connected if a signal output from X istransmitted to Y.

Note that when it is explicitly described that X and Y are electricallyconnected, the case where X and Y are electrically connected (i.e., thecase where X and Y are connected with another element or another circuitprovided therebetween), the case where X and Y are functionallyconnected (i.e., the case where X and Y are functionally connected withanother circuit provided therebetween), and the case where X and Y aredirectly connected (i.e., the case where X and Y are connected withoutanother element or another circuit provided therebetween) are includedtherein. That is, the explicit expression “X and Y are electricallyconnected” is the same as the explicit simple expression “X and Y areconnected”.

For example, any of the following expressions can be used for the casewhere a source (or a first terminal or the like) of a transistor iselectrically connected to X through (or not through) Z1 and a drain (ora second terminal or the like) of the transistor is electricallyconnected to Y through (or not through) Z2, or the case where a source(or a first terminal or the like) of a transistor is directly connectedto one part of Z1 and another part of Z1 is directly connected to Xwhile a drain (or a second terminal or the like) of the transistor isdirectly connected to one part of Z2 and another part of Z2 is directlyconnected to Y.

The expressions include, for example, “X, Y, a source (or a firstterminal or the like) of a transistor, and a drain (or a second terminalor the like) of the transistor are electrically connected to each other,and X, the source (or the first terminal or the like) of the transistor,the drain (or the second terminal or the like) of the transistor, and Yare electrically connected to each other in this order”, “a source (or afirst terminal or the like) of a transistor is electrically connected toX, a drain (or a second terminal or the like) of the transistor iselectrically connected to Y, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are electrically connected to each otherin this order”, and “X is electrically connected to Y through a source(or a first terminal or the like) and a drain (or a second terminal orthe like) of a transistor, and X, the source (or the first terminal orthe like) of the transistor, the drain (or the second terminal or thelike) of the transistor, and Y are provided to be connected in thisorder”. When the connection order in a circuit configuration is definedby an expression similar to the above examples, a source (or a firstterminal or the like) and a drain (or a second terminal or the like) ofa transistor can be distinguished from each other to specify thetechnical scope. Note that these expressions are examples and there isno limitation on the expressions. Here, X, Y, Z1, and Z2 each denote anobject (e.g., a device, an element, a circuit, a wiring, an electrode, aterminal, a conductive film, and a layer).

Even when independent components are electrically connected to eachother in a circuit diagram, one component has functions of a pluralityof components in some cases. For example, when part of a wiring alsofunctions as an electrode, one conductive film functions as the wiringand the electrode. Thus, “electrical connection” in this specificationincludes in its category such a case where one conductive film hasfunctions of a plurality of components.

«Parallel and Perpendicular»

In this specification, the term “parallel” indicates that the angleformed between two straight lines is greater than or equal to −100 andless than or equal to 100, and accordingly also includes the case wherethe angle is greater than or equal to −5° and less than or equal to 5°.In addition, the term “substantially parallel” indicates that the angleformed between two straight lines is greater than or equal to −30° andless than or equal to 300. The term “perpendicular” indicates that theangle formed between two straight lines is greater than or equal to 800and less than or equal to 1000. Thus, the case where the angle isgreater than or equal to 850 and less than or equal to 950 is alsoincluded. In addition, the term “substantially perpendicular” indicatesthat the angle formed between two straight lines is greater than orequal to 600 and less than or equal to 1200.

«Trigonal and Rhombohedral»

In this specification, trigonal and rhombohedral crystal systems areincluded in a hexagonal crystal system.

EXPLANATION OF REFERENCE

OUT[1]: column output circuit, OUT[j]: column output circuit, OUT[n]:column output circuit, OUT[j+1]: column output circuit, C_(ref):reference column output circuit, AM[1,1]: memory cell, AM[i,1]: memorycell, AM[m,1]: memory cell, AM[1,j]: memory cell, AM[i,j]: memory cell,AM[m,j]: memory cell, AM[1,n]: memory cell, AM[i,n]: memory cell,AM[m,n]: memory cell, AM[i+1,j]: memory cell, AM[i,j+1]: memory cell,AM[i+,j+1]: memory cell, AM_(ref)[1]: memory cell, AM_(ref)[i]: memorycell, AM_(ref)[m]: memory cell, AM_(ref)[i+1]: memory cell, CI: constantcurrent circuit, CI_(ref): constant current circuit, CM: current mirrorcircuit, OT[1]: output terminal, OT[j]: output terminal, OT[n]: outputterminal, OT[i+1]: output terminal, OT_(ref): output terminal, SPT[1]:output terminal, SPT[j]: output terminal, SPT[n]: output terminal,SPT[j+1]: output terminal, CT1: terminal, CT2: terminal, CT3: terminal,CT4: terminal, CT5[1]: terminal, CT5[j]: terminal, CT5[n]: terminal,CT5[j+1]: terminal, CT6[1]: terminal, CT6[j]: terminal, CT6[n]:terminal, CT6[j+1]: terminal, CT7: terminal, CT8: terminal, Tr1:transistor, Tr2: transistor, Tr3: transistor, Tr4: transistor, Tr5:transistor, Tr6: transistor, Tr7: transistor, Tr11: transistor, Tr12:transistor, C1: capacitor, C2: capacitor, OSP: wiring, ORP: wiring,B[1]: wiring, B[j]: wiring, B[n]: wiring, B[i+1]: wiring, B_(ref):wiring, WD[1]: wiring, WD[j]: wiring, WD[n]: wiring, WD[j+1]: wiring,WD_(ref): wiring, VR: wiring, RW[1]: wiring, RW[i]: wiring, RW[m]:wiring, RW[i+1]: wiring, WW[1]: wiring, WW[i]: wiring, WW[m]: wiring,WW[i+1]: wiring, OL[1]: wiring, OL[j]: wiring, OL[n]: wiring, OL[j+1]:wiring, OL_(ref): wiring, IL[1]: wiring, IL[j]: wiring, IL[n]: wiring,IL[j+1]: wiring, IL_(ref): wiring, BG[1]: wiring, BG[j]: wiring, BG[n]:wiring, BG[j+1]: wiring, BG_(ref): wiring, VDDL: wiring, VSSL: wiring,NCM_(ref): node, N[1,1]: node, N[i,1]: node, N[m,1]: node, N[1,j]: node,N[i,j]: node, N[m,j]: node, N[1,n]: node, N[i,n]: node, N[m,n]: node,N[i+1,j]: node, N[i,j+1]: node, N[i+1,j+1]: node, N_(ref)[1]: node,N_(ref)[i]: node, N_(ref)[m]: node, N_(ref)[i+l]: node, Co1: arrow, Co2:arrow, Ro1: arrow, Ro2: arrow, SW1: switch, SW2: switch, M: transistor,MD: transistor, C11: capacitor, C12: capacitor, Sig1(i): signal line,Sig2(i): signal line, Sig1(i+1): signal line, Sig2(i+1): signal line,G1(i): scan line, G2(i): scan line, CL(g): control line, ML(h): sensorsignal line, C(g): electrode, M(h): electrode, BR(g,h): conductive film,CSCOM: wiring, VCOM1: wiring, VCOM2: fourth conductive film, ANO: thirdconductive film, FPC1: flexible printed circuit, FPC2: flexible printedcircuit, ACF1: conductive material, ACF2: conductive material, AF1:alignment film, AF2: alignment film, BM: light-blocking film, CFT:coloring film, CF2: coloring film, KBT: structure body, CP: conductivematerial, GD: driver circuit, SD: driver circuit, OSC: oscillatorcircuit, DC: detection circuit, I1: insulator, I2: insulator, S1: oxide,S2: oxide, S3: oxide, 100: semiconductor device, 110: offset circuit,111: offset circuit, 112: offset circuit, 113: offset circuit, 120:memory cell array, 121: memory cell array, 150: offset circuit, 160:memory cell array, 501A: first insulating film, 501C: second insulatingfilm, 504: conductive film, 506: insulating film, 505: bonding layer,508: semiconductor film, 511B: conductive film, 511C: conductive film,512A: conductive film, 512B: conductive film, 516: insulating film, 518:insulating film, 519B: terminal, 519C: terminal, 520: functional layer,521: insulating film, 522: connection portion, 524: conductive film,528: insulating film, 530(i,j): pixel circuit, 530(i,j+1): pixelcircuit, 550(i,j): second display element, 550(i,j+1): second displayelement, 551(i,j): third electrode, 552: fourth electrode, 553(j):layer, 570: substrate, 591A: opening, 591B: opening, 591C: opening,592A: first opening, 592B: second opening, 592C: opening, 700TP1: touchpanel, 702(i,j): pixel, 702(i,j+1): pixel, 702(i+1,j): pixel,702(i+2,j): pixel, 705: sealing material, 706: insulating film, 709:bonding layer, 710: substrate, 719: terminal, 750(i,j): first displayelement, 750(i,j+1): first display element, 750(i,j+2): first displayelement, 751(i,j): first electrode, 751(i,j+1): first electrode,751(i,j+2): first electrode, 751(i+1,j): first electrode, 751(i+2,j):first electrode, 751E: region, 751H: opening, 752: second electrode,753: layer, 754A: first intermediate film, 754B: second intermediatefilm, 754C: intermediate film, 770: substrate, 770P: functional layer,770D: functional layer, 771: insulating film, 775(g,h): sensing element,775(g,1): sensing element, 775(g,q): sensing element, 775(1,h): sensingelement, 775(p,h): sensing element, 1200A: transistor, 1200B:transistor, 1200C: transistor, 1200D: transistor, 1200E: transistor,1200F: transistor, 1200G: transistor, 1205: conductor, 1205 a:conductor, 1205 b: conductor, 1220: insulator, 1222: insulator, 1224:insulator, 1230: oxide, 1230 a: oxide, 1230 b: oxide, 1230 c: oxide,1230 d: oxide, 1240 a: conductor, 1240 b: conductor, 1241 a: conductor,1241 b: conductor, 1250: insulator, 1260: conductor, 1260 a: conductor,1260 b: conductor, 1260 c: conductor, 1270: insulator, 1280: insulator,1282: insulator, 1285: insulator, 1286: insulator, 4700: electroniccomponent, 4701: lead, 4702: printed circuit board, 4703: circuitportion, 4704: circuit board, 4800: semiconductor wafer, 4800 a: chip,4801: wafer, 4801 a: wafer, 4802: circuit portion, 4803: spacing, 4803a: spacing, 4810: semiconductor wafer, 5221: housing, 5222: displayportion, 5223: operation button, 5224: speaker, 5431: housing, 5432:display portion, 5433: palm print reading portion, 5434: wiring, 5435:hand, 5501: housing, 5502: display portion, 5503: microphone, 5504:speaker, 5505: operation button, 5801: first housing, 5802: secondhousing, 5803: display portion, 5804: operation key, 5805: lens, 5806:joint, 6560: TV, 6561: broadcast station, 6562: artificial satellite,6563: radio wave tower, 6564: antenna, 6565: antenna, 6566A: radio wave,6566B: radio wave, 6567A: radio wave, 6567B: radio wave, 6600:ambulance, 6601: medical institution, 6602: medical institution, 6605:high-speed network, 6610: camera, 6611: encoder, 6612: communicationdevice, 6615: video data, 6616: video data, 6620: communication device,6621: decoder, 6622: server, 6623: display device

This application is based on Japanese Patent Application serial no.2016-055281 filed with Japan Patent Office on Mar. 18, 2016, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A semiconductor device comprising: a memorycell array; and an offset circuit, wherein the memory cell arraycomprises a first memory cell and a second memory cell, wherein theoffset circuit comprises a first constant current circuit, a secondconstant current circuit, first to third transistors, a first capacitor,a first wiring, a second wiring, a first output terminal, a secondoutput terminal, and a current mirror circuit, wherein the firstconstant current circuit is electrically connected to the first wiring,wherein the first constant current circuit is configured to supply afirst current to the first wiring, wherein a first terminal of the firsttransistor is electrically connected to a first terminal of the secondtransistor, wherein a gate of the first transistor is electricallyconnected to a second terminal of the second transistor, wherein thefirst terminal of the second transistor is electrically connected to thefirst wiring, wherein a first terminal of the third transistor iselectrically connected to the second terminal of the second transistor,wherein a first terminal of the first capacitor is electricallyconnected to the gate of the first transistor, wherein the first wiringis electrically connected to the first output terminal, wherein thesecond constant current circuit is electrically connected to the secondwiring, wherein the second constant current circuit is configured tosupply a second current to the second wiring, wherein the second wiringis electrically connected to the second output terminal, wherein thecurrent mirror circuit is configured to output a third currentcorresponding to a potential of the second wiring from the first wiringand the second wiring, wherein the first memory cell is electricallyconnected to the first output terminal, and wherein the second memorycell is electrically connected to the second output terminal.
 2. Thesemiconductor device according to claim 1, wherein the first constantcurrent circuit comprises a fourth transistor, wherein the secondconstant current circuit comprises a fifth transistor, wherein thefourth transistor and the fifth transistor each comprise a back gate,wherein a first terminal of the fourth transistor is electricallyconnected to the first wiring, wherein a gate of the fourth transistoris electrically connected to the first terminal of the fourthtransistor, wherein a first terminal of the fifth transistor iselectrically connected to the second wiring, and wherein a gate of thefifth transistor is electrically connected to the first terminal of thefifth transistor.
 3. The semiconductor device according to claim 1,wherein the current mirror circuit comprises a sixth transistor and aseventh transistor, wherein a first terminal of the sixth transistor iselectrically connected to the first wiring, wherein a gate of the sixthtransistor is electrically connected to the second wiring, wherein afirst terminal of the seventh transistor is electrically connected tothe second wiring, and wherein a gate of the seventh transistor iselectrically connected to the second wiring.
 4. The semiconductor deviceaccording to claim 1, wherein the first memory cell comprises an eighthtransistor, a ninth transistor, and a second capacitor, wherein thesecond memory cell comprises a tenth transistor, an eleventh transistor,and a third capacitor, wherein a first terminal of the eighth transistoris electrically connected to a gate of the ninth transistor, wherein afirst terminal of the second capacitor is electrically connected to thefirst terminal of the eighth transistor, wherein a first terminal of theninth transistor is electrically connected to the first output terminal,wherein a first terminal of the tenth transistor is electricallyconnected to a gate of the eleventh transistor, wherein a first terminalof the third capacitor is electrically connected to the first terminalof the tenth transistor, and wherein a first terminal of the eleventhtransistor is electrically connected to the second output terminal. 5.The semiconductor device according to claim 4, wherein the first toeleventh transistors each have a same polarity.
 6. The semiconductordevice according to claim 4, wherein at least one of channel formationregions in the first to eleventh transistors comprises an oxidecontaining at least one of indium, an element M, and zinc, and whereinthe element M is any one of aluminum, gallium, yttrium, and tin.
 7. Asemiconductor wafer comprising: a plurality of the semiconductor devicesaccording to claim 1; and a region for dicing.
 8. An electronic devicecomprising: the semiconductor device according to claim 1; and ahousing.
 9. An electronic device comprising: the semiconductor deviceaccording to claim 1 and is configured to perform processing such aspattern recognition and associative storage.
 10. A biometricauthentication system comprising: an electronic device comprising asemiconductor device and is configured to perform processing such aspattern recognition and associative storage; and a device configured toobtain biological information, wherein the semiconductor devicecomprises a memory cell array and an offset circuit, wherein the memorycell array comprises a first memory cell and a second memory cell,wherein the offset circuit comprises a first constant current circuit, asecond constant current circuit, first to third transistors, a firstcapacitor, a first wiring, a second wiring, a first output terminal, asecond output terminal, and a current mirror circuit, wherein the firstconstant current circuit is electrically connected to the first wiring,wherein the first constant current circuit is configured to supply afirst current to the first wiring, wherein a first terminal of the firsttransistor is electrically connected to a first terminal of the secondtransistor, wherein a gate of the first transistor is electricallyconnected to a second terminal of the second transistor, wherein thefirst terminal of the second transistor is electrically connected to thefirst wiring, wherein a first terminal of the third transistor iselectrically connected to the second terminal of the second transistor,wherein a first terminal of the first capacitor is electricallyconnected to the gate of the first transistor, wherein the first wiringis electrically connected to the first output terminal, wherein thesecond constant current circuit is electrically connected to the secondwiring, wherein the second constant current circuit is configured tosupply a second current to the second wiring, wherein the second wiringis electrically connected to the second output terminal, wherein thecurrent mirror circuit is configured to output a third currentcorresponding to a potential of the second wiring from the first wiringand the second wiring, wherein the first memory cell is electricallyconnected to the first output terminal, and wherein the second memorycell is electrically connected to the second output terminal.
 11. Thebiometric authentication system according to claim 10, wherein the firstconstant current circuit comprises a fourth transistor, wherein thesecond constant current circuit comprises a fifth transistor, whereinthe fourth transistor and the fifth transistor each comprise a backgate, wherein a first terminal of the fourth transistor is electricallyconnected to the first wiring, wherein a gate of the fourth transistoris electrically connected to the first terminal of the fourthtransistor, wherein a first terminal of the fifth transistor iselectrically connected to the second wiring, and wherein a gate of thefifth transistor is electrically connected to the first terminal of thefifth transistor.
 12. The biometric authentication system according toclaim 10, wherein the current mirror circuit comprises a sixthtransistor and a seventh transistor, wherein a first terminal of thesixth transistor is electrically connected to the first wiring, whereina gate of the sixth transistor is electrically connected to the secondwiring, wherein a first terminal of the seventh transistor iselectrically connected to the second wiring, and wherein a gate of theseventh transistor is electrically connected to the second wiring. 13.The biometric authentication system according to claim 10, wherein thefirst memory cell comprises an eighth transistor, a ninth transistor,and a second capacitor, wherein the second memory cell comprises a tenthtransistor, an eleventh transistor, and a third capacitor, wherein afirst terminal of the eighth transistor is electrically connected to agate of the ninth transistor, wherein a first terminal of the secondcapacitor is electrically connected to the first terminal of the eighthtransistor, wherein a first terminal of the ninth transistor iselectrically connected to the first output terminal, wherein a firstterminal of the tenth transistor is electrically connected to a gate ofthe eleventh transistor, wherein a first terminal of the third capacitoris electrically connected to the first terminal of the tenth transistor,and wherein a first terminal of the eleventh transistor is electricallyconnected to the second output terminal.
 14. The biometricauthentication system according to claim 13, wherein the first toeleventh transistors each have a same polarity.
 15. The biometricauthentication system according to claim 13, wherein at least one ofchannel formation regions in the first to eleventh transistors comprisesan oxide containing at least one of indium, an element M, and zinc, andwherein the element M is any one of aluminum, gallium, yttrium, and tin.16. A video distribution system comprising: an electronic devicecomprising a semiconductor device and is configured to performprocessing such as pattern recognition and associative storage, whereinthe video distribution system is configured to encode a video data, andwherein the video distribution system is configured to send an encodedvideo data, wherein the semiconductor device comprises a memory cellarray and an offset circuit, wherein the memory cell array comprises afirst memory cell and a second memory cell, wherein the offset circuitcomprises a first constant current circuit, a second constant currentcircuit, first to third transistors, a first capacitor, a first wiring,a second wiring, a first output terminal, a second output terminal, anda current mirror circuit, wherein the first constant current circuit iselectrically connected to the first wiring, wherein the first constantcurrent circuit is configured to supply a first current to the firstwiring, wherein a first terminal of the first transistor is electricallyconnected to a first terminal of the second transistor, wherein a gateof the first transistor is electrically connected to a second terminalof the second transistor, wherein the first terminal of the secondtransistor is electrically connected to the first wiring, wherein afirst terminal of the third transistor is electrically connected to thesecond terminal of the second transistor, wherein a first terminal ofthe first capacitor is electrically connected to the gate of the firsttransistor, wherein the first wiring is electrically connected to thefirst output terminal, wherein the second constant current circuit iselectrically connected to the second wiring, wherein the second constantcurrent circuit is configured to supply a second current to the secondwiring, wherein the second wiring is electrically connected to thesecond output terminal, wherein the current mirror circuit is configuredto output a third current corresponding to a potential of the secondwiring from the first wiring and the second wiring, wherein the firstmemory cell is electrically connected to the first output terminal, andwherein the second memory cell is electrically connected to the secondoutput terminal.